HOW   TO   MAKE   A 

WIRELESS  SET 


BY 


ARTHUR   MOORE 


POPULAR  MECHANICS  HANDBOOKS 


CHICAGO 
POPULAR    MECHANICS  CO 


Copyright,  1911, 
by  H.  H.  WINDSOR 


THIS  book  is  one  of  the  series  of 
handbooks  on  industrial  subjects 
being  published  by  the  Popular 
Mechanics    Company.      Like   the 
Magazine,   these  books  are  "written 
so  you  can    understand    it,"  and  are 
intended   to    furnish    information   on 
mechanical  subjects  at  a  price  within 
the  reach  of  all. 

The  text  and  illustrations  have 
been  prepared  expressly  for  this 
Handbook  Series,  by  experts;  are 
up-to-date,  and  have  been  revised  by 
the  editor  of  Popular  Mechanics. 


CONTENTS 


PAGE 

Chapter         I — The  Necessary  Parts 9 

Chapter       II— The  Spark  Coil 15 

Chapter     III — The  Condenser 22 

Chapter     IV— The  Spark  Gap  and  Vibrator 29 

Chapter       V — Construction  of  Telegraph  Key  and 

Aerial  Wires 37 

Chapter     VI — Construction    of    a    Coherer   and    a 

Decoherer 43 

Chapter    VII — Construction  of  a  Potentiometer  ...  51 

Chapter  VIII — Secondary  Windings 59 

Chapter     IX — Another  Secondary  Winding 67 

Chapter       X — Connections  for  Sets 77 


HOW  TO   MAKE  A 

WIRELESS   SET 

CHAPTER  I 
THE  NECESSARY  PARTS 

THERE  is  no  field  of  electrical  experimentation 
that  has  received  more  attention  from  the  ama- 
teur in  the  last  few  years  than  that  of  wireless  teleg- 
raphy. A  great  many  persons  have  the  impression 
that  one  must  be  an  electrical  engineer  in  order  to  con- 
struct and  operate  successfully  a  wireless  telegraph 
set.  Such  an  impression  is  not  well  grounded.  The 
following  description  of  a  small  wireless  set  will  not 
only  be  useful  to  those  who  wish  to  experiment  along 
this  particular  line,  but  it  will  be  a  great  aid  in  making 
clear  the  elementary  principles  upon  which  wireless 
telegraphy  is  based.  A  set  suitable  for  transmitting 
for  distances  of  four  or  five  miles  in  the  open  country 
can  be  easily  constructed  by  a  boy  of  ordinary  abil- 
ity at  a  small  cost.  The  information  gained  by  con- 
structing a  small  set  would  be  a  great  help  to  those 
who  intend  building  larger  outfits  later. 

Before  explaining  the  construction  of  the  various 
parts  in  detail,  it  might  "be  well  to  give  a  general  de- 
scription of  them  and  explain  their  functions  and  ope- 
ration. You  will  need  the  following  parts  to  complete 
the  equipment  for  a  single  station  that  can  both  send 
and  receive  a  message :  A  spark  coil  that  is  capable  of 
producing  a  spark  from  1  to  1^/2  in.  long,  telegraph 


HOW  TO  MAKE  A  WIRELESS  SET  11 

key,  150  or  200-ohm  relay,  common  vibrating  electric 
bell,  aerial  conductors,  coherer  and  spark  gap. 

The  spark  coil  is  used  to  produce  a  very  high  voltage 
current  from  a  source  of  electrical  energy  of  low  volt- 
age, such  as  dry  cells,  and  consists  of  two  windings 
about  an  iron  core. 

The  source  of  electrical  energy  is  connected  in  series 
with  one  of  these  windings,  which  is  called  the  "pri- 
mary."' The  terminals  of  the  other  winding  are  con- 
nected to  the  spark  gap,  as  shown  in  Fig.  1.  This  sec- 
ond winding  is  called  the  "secondary."  There  will 
be  produced  in  the  secondary  winding  an  electromo- 
tive force  when  there  is  a  change  of  current  in  the  pri- 
mary. If  the  primary  circuit  is  then  made  and  broken 
by  placing  the  telegraph  key  in  circuit  with  it,  there 
will  be  an  electromotive  force  set  up  in  the  secondary 
for  each  make  and  break  in  the  primary  due  to  the 
operation  of  the  key.  The  value  of  the  induced  elec- 
tromotive force  in  the  secondary  will  depend  upon 
the  ratio  of  primary  and  secondary  turns  and  the  ra- 
pidity of  the  "making  and  breaking"  of  the  primary 
circuit  which  can  be  greatly  increased  by  placing  in 
the  circuit  a  current  "breaker,"  or  interrupter,  as  it  is 
more  usually  termed.  The  construction  of  the  inter- 
rupter is  indicated  in  Fig.  1.  When  the  key  K  is 
closed,  the  iron  core  ll  becomes  magnetized,  due  to 
the  current  in  the  primary,  and  as  a  result  attracts  the 
iron  armature  I,  which  is  normally  held  away  from 
the  core  by  the  spring  G.  The  movement  of  the  ar- 
mature I  toward  the  core  Ilf  results  in  the  contact  at 
C  being  broken  and  the  current  in  the  primary  wind- 
ing dropping  to  zero.  The  iron  core  loses  its  magnet- 
ism and  the  armature  again  returns  to  its  normal  po- 
sition, completing  the  circuit  at  C  and  again  going 
through  the  same  operation.  With  this  interrupter  in 


12  HOW  TO  MAKE  A  WIRELESS  SET 

circuit,  as  shown  in  Fig.  1,  there  will  be  a  number  of 
makes  and  breaks  in  the  primary  when  the  key  is 
closed  only  for  a  short  time  and  hence  a  much  greater 
electromotive  force  set  up  in  the  secondary. 

There  is  quite  an  arc  formed  at  the  contact  C  when 
the  circuit  is  broken,  due  to  the  inductance  of  the  pri- 
mary winding.  The  condenser  D  is  connected  directly 
across  the  gap  and  reduces  to  a  great  extent  the  ten- 
dency for  the  arc  to  form,  and  as  a  result,  the  decay  of 
the  primary  current  is  a  great  deal  more  rapid,  hence 
a  greater  electromotive  force  is  set  up  in  the  second- 
ary, and  it  will  be  continuous  as  long  as  the  key  is 
closed. 

One  terminal  of  the  spark  gap  is  well  grounded  and 
the  other  terminal  is  connected  to  the  aerial  as  indi- 
cated in  Fig.  1.  The  aerial  may  be  made  from  a 
piece  of  Xo.  14  gauge  copper  wire  that  has  one  end 
fastened  to  the  top  of  a  GO  or  70-ft.  pole  by  means  of 
two  knob  insulators.  One  of  these  knobs  can  be  tied 
to  a  rope  that  runs  in  a  small  pulley  on  the  top  of  the 
pole ;  one  end  of  the  aerial  is  fastened  to  the  other  in- 
sulator or  knob,  and  the  two  knobs  are  then  connected 
by  a  link  of  wire.  The  aerial  should  be  run  off  at  an 
angle  to  the  pole  to  prevent  them  touching,  and  it 
should  be  well  insulated  where  it  is  led  into  the  build- 
ing. If  it  is  possible,  the  pole  carrying  the  aerial 
should  be  placed  on  the  top  of  a  building.  This  will 
give  much  better  results.  A  good  ground  connection 
can  be  made  by  soldering  a  No.  14  gauge  wire  to  a 
metal  plate  and  burying  it  at  a  depth  of  5  or  G  ft.  in 
moist  earth.  The  sending  end  is  now  complete.  The 
details  in  construction  and  adjustment  will  be  taken 
up  later. 

The  coherer  is  the  most  important  part  of  the  re- 
ceiving set.  Branly  was  the  first  to  discover  that  a 


HOW  TO  MAKE  A  WIRELESS  SET  13 

quantity  of  iron  filings  when  brought  within  the  in- 
fluence of  a  high-frequency  discharge,  such  as  that 
from  an  induction  coil  or  Leyden  jar,  will  cling  to- 
gether, or  cohere  as  it  is  termed,  thus  lowering  the  re- 
sistance of  the  quantity  of  iron  filings  as  a  whole,  and 
a  current  from  a  battery  will  more  easily  flow  through 
them  when  they  are  in  coherence  than  when  they  are 
not.  These  filings  are  usually  placed  in  a  glass  tube 
between  two  metal  terminals.  One  of  these  terminals 
is  connected  to  the  aerial  and  the  other  is  grounded, 
when  the  station  is  used  in  receiving,  as  in  Fig.  2. 

A  battery  of  low  voltage  and  the  relay  are  con- 
nected across  the  terminals  of  the  coherer,  as  shown  in 
Fig.  2.  The  adjustment  of  the  coherer  and  the  relay 
is  such  that  there  is  not  sufficient  current  through  the 
relay  to  operate  it  when  the  filings  are  not  cohered. 
When,  however,  the  coherer  is  acted  upon  by  a  high- 
frequency  discharge,  the  filings  will  cohere,  thus  re- 
ducing the  resistance  of  the  relay  circuit  and  increas- 
ing the  current  through  the  relay,  which  will  operate 
it,  if  the  proper  adjustment  has  been  made. 

This  relay  in  Fig.  2  can  be  made  to  close  a  second 
local  circuit  consisting  of  a  battery  of  a  few  dry  cells 
and  an  ordinary  vibrating  bell,  which  serves  to  give 
an  audible  signal.  The  relay  consists  of  two  magnet 
coils  connected  in  series  and  their  outside  terminals 
connected  to  the  posts  P!  and  P2,  Fig.  2.  There  is  a 
soft  iron  armature  that  is  delicately  balanced  on  two 
pivots  near  the  ends  of  the  cores  of  these  two  coils. 
These  two  iron  cores  become  sufficiently  magnetized, 
due  to  a  very  small  current  in  the  coils,  to  attract  the 
armature,  which  is  normally  held  away  from  them  a 
very  small  distance  by  a  weak  coil  spring.  The  arma- 
ture closes  the  second  circuit  by  coming  in  contact 
with  the  point  P3. 


14  HOW  TO  MAKE  A  WIRELESS  SET 

When  the  iron  filings  are  once  cohered  they  will 
not  return  to  their  original  condition  if  they  are  re- 
moved from  the  influence  of  the  high-frequency  dis- 
charge, and  as  a  result  the  bell  would  continue  to 
ring  even  though  the  coherer  was  not  being  acted 
upon  by  any  discharge.  To  overcome  this  objection- 
able feature  the  filings  must  be  decohered,  which  can 
be  done  by  allowing  the  tapper  of  the  electric  bell  to 
strike  lightly  against  the  glass  tube  of  the  coherer. 
The  receiving  station  is  now  complete  and  the  opera- 
tion of  such  a  set  can  be  traced  as  follows :  The  tele- 
graph key  at  the  sending  end  is  pressed,  allowing  a 
current  to  flow  through  the  primary  winding  and 
start  the  vibrator  in  operation.  At  each  make  and 
break  of  the  primary  circuit  due  to  the  vibrator,  there 
will  be  a  high  voltage  induced  in  the  secondary  which 
is  sufficient  to  break  down  the  air  gap  between  the  ter- 
minals of  the  spark  gap,  and  as  a  result,  a  high-fre- 
quency current  will  flow  in  the  secondary  circuit. 
This  high-frequency  current  oscillates  up  and  down 
the  aerial  and  is  supposed  to  set  up  a  similar  motion 
in  the  ether  existing  in  the  air  and  the  ground.  This 
wave  motion  is  sent  out  in  all  directions  and  any  re- 
ceiving station  that  is  within  its  influence  receives  a 
small  portion  of  the  total  energy  sent  out.  As  a  re- 
sult a  very  high  frequency  current  flows  through  the 
coherer,  thus  reducing  its  resistance  and  operating  the 
relay  and  closing  the  local  circuit  which  starts  the 
bell  in  operation.  The  bell  will  continue  to  ring  as 
long  as  the  wave  continues  to  come  in,  which  is  de- 
termined by  the  time  the  telegraph  key  at  the  sending 
end  is  closed,  and  as  a  result  the  dots  and  dashes  of 
the  code  can  be  easily  distinguished. 


CHAPTER   II 

THE  SPARK  COIL 

THE  spark  coil  is  the  principal  piece  of  apparatus 
used  in  the  transmitter  of  a  wireless  telegraph  set 
and  its  detailed  construction  is  given  herewith.  The 
coil  described  here  can  be  used  for  ordinary  laboratory 
work,  lighting  Geissler  tubes,  etc.,  in  addition  to  its 
use  in  the  wireless  set.  All  spark  coils  might  be 
thought  of  as  composed  of  the  following  parts:  iron 
wire  core,'  primary  winding,  secondary  winding,  vi- 
brator, condenser  and  containing  case.  These  vari- 
ous parts  will  be  taken  up  in  turn. 

The  core  of  a  spark  coil  is  one  of  the  most  impor- 
tant parts  and  it  should  be  constructed  of  a  large  num- 
ber of  small  soft  iron  wires.  If  a  solid  iron  core  is 
used,  the  reversal  of  magnetism  in  the  core  does  not 
take  place  as  rapidly  as  the  current  is  interrupted  by 
the  vibrator.  That  is,  the  solid  iron  core  would  not 
take  on  and  lose  its  magnetic  effect  as  rapidly  as  the 
vibrator  worked,  which  would  result  in  a  weak  cur- 
rent in  the  secondary.  A  core  composed  of  a  number 
of  small  iron  wires  is  said  to  be  laminated. 

To  build  the  core,  procure  a  sufficient  quantity  of 
No.  22  gauge  soft  iron  wire  to  form  a  bundle  1  in.  in 
diameter  and  10  in.  long.  All  of  these  pieces  should 
be  cut  to  the  same  length  and  straightened  before 
they  are  placed  in  the  bundle.  One  easy  way  to  form 
this  core  is  to  wind  on  a  stick  1  in.  in  diameter  at  least 
six  turns  of  good  quality  paper,  and  glue  the  various 
turns  as  they  are  rolled  on,  forming  a  tube.  Fill  this 

15 


HOW  TO  MAKE  A  WIRELESS  SET  17 

tube  with  the  pieces  of  iron  wire,  then  roll  it  between 
two  boards  which  will  cause  the  wires  to  imbed  them- 
selves better,  and  you  can  add  more  pieces,  making 
your  core  much  more  compact.  Wrap  this  core  with 
a  good  quality  twine,  such  as  is  used  by  shoemakers, 
the  various  turns  to  be  placed  neatly  together. 

Outside  of  the  twine,  wrap  15  or  20  layers  of  very 
porous  tissue  paper  and  boil  the  whole  core  in  a  hot 
bath  of  beeswax  and  paraffin  for  at  least  one  hour. 
This  completes  the  core  and  you  can  now  wind  upon 
it  the  primary  winding.  The  primary  winding  is  to 
consist  of  three  layers  of  No.  16  gauge  double  cotton- 
covered  magnet  wire  wound  on  the  core  to  within  1 
in.  of  each  end.  It  might  be  well  to  place  two  or  three 
turns  of  good  quality  paper  between  the  various  lay- 
ers. A  terminal  at  least  10  in.  long  should  be  left  at 
each  end  for  making  connections  to  the  coil.  It  might 
be  well  for  you  to  place  a  piece  of  small  rubber  tub- 
ing over  the  terminals  where  they  are  led  from  the 
winding.  The  completed  core  and  winding,  as  shown 
in  Fig.  1,  should  be  thoroughly  boiled  in  beeswax  and 
paraffin  before  starting  on  the  secondary  winding. 

The  secondary  winding  is  to  be  placed  outside  of 
this  primary  winding,  but  must  be  insulated  from  it 
by  an  insulating  tube  placed  over  the  primary  wind- 
ing. You  can  construct  such  a  tube  by  winding  on  a 
piece  of  wood,  whose  outside  diameter  is  a  little 
greater  than  the  outside  diameter  of  the  primary 
winding,  several  layers  of  paper  and  afterwards  boil- 
ing it  in  paraffin.  This  tube  should  be  about  1  in. 
longer  than  the  primary  winding  and  the  inside  diam- 
eter should  be  a  neat  fit  over  it.  The  wall  should  be 
about  W  in.  in  thickness.  All  the  additional  space 
between  the  primary  winding  and  the  tube  should  be 
filled  with  paraffin. 


18  HOW  TO  MAKE  A  WIRELESS  SET 

Too  much  care  cannot  be  used  in  properly  con- 
structing and  insulating  the  secondary  winding.  For 
very  small  coils,  it  is  customary  to  wind  all  of  the  sec- 
ondary wire  in  one  section,  while  in  larger  coils,  the 
secondary  winding  is  composed  of  a  number  of  sec- 
tions, if'it  is  desired  to  reduce  the  possibility  of  break- 
downs. When  all  of  the  wire  is  wound  in  one  section, 
the  difference  in  electrical  pressure  between  the  vari- 
ous layers  becomes  enormous  and  the  insulation  must 
be  greatly  increased.  If  the  secondary  is  made  of  a 
number  of  sections,  as  shown  in  Fig.  2,  and  they  are 
so  connected  that  their  electromotive  forces  all  act 
in  the  same  direction,  the  same  total  electromotive 
force  can  be  obtained  from  the  coil,  but  the  voltage 
between  the  various  parts  of  the  winding  is  not  nearly 
so  great  as  in  the  previous  case. 

The  secondary  of  the  coil  described  here  is  to  con- 
sist of  six  double  sections,  and  these  six  sections  when 
placed  side  by  side  should  take  up  a  length  of  6  in., 
that  is,  each  section  should  be  1  in.  in  thickness.  You 
will  find  the  winding  of  these  various  sections  of  the 
secondary  a  very  tedious  job  if  attempted  by  hand, 
and  time  will  be  gained  by  constructing  a  simple  form 
of  winding  machine. 

The  core  of  the  form  upon  which  these  coils  are  to 
be  wound  should  have  a  diameter  a  little  greater  than 
the  outside  diameter  of  the  insulating  tube.  The  cross 
section  of  such  a  form  is  shown  in  Fig.  3.  The  disks 
Dj,  D2  and  D3  can  be  made  of  %-in.  wood,  and  should 
be  about  4  in,  outside  diameter.  The  two  cores  Q 
and  C2  should  have  a  length  of  about  %  in.,  and  the 
diameter  of  the  two  ends  should  differ  by  iV  to  %  in- 
This  form  can  be  fastened  in  a  lathe  and  the  wire 
wound  on  in  a  very  short  time,  but  usually  the  ama- 
teur has  no  lathe  that  can  be  used  and  must  devise 


HOW  TO  MAKE  A  WIRELESS  SET  19 

some  type  of  machine  that  will  serve  the  same  pur- 
pose. The  form  just  described  can  be  supported  by 
two  uprights  as  shown  in  Fig.  4  and  it  can  be  rotated 
by  means  of  a  small  handle  attached  to  one  end  of  the 
shaft. 

In  winding  the  secondary,  the  wire  should  pass 
through  a  hot  bath  of  beeswax  and  paraffin  as  it  is 
being  placed  on  the  coil,  instead  of  depending  upon 
the  boiling-out  process,  as  in  the  case  of  the  primary 
winding.  This  can  be  accomplished  by  the  arrange- 
ment shown  in  Fig.  5.  A  small  can,  E,  is  supported 
above  a  gas  or  oil  flame,  L,  and  contains  the  beeswax 
and  paraffin.  The  wire  is  made  to  pass  through  this 
mixture  because  it  must  pass  over  the  small  pulley 
P,  which  is  below  the  surface  of  the  mixture.  The 
tension  in  the  wire  can  be  adjusted  by  placing  a  small 
brake,  B,  on  the  spool  from  which  the  wire  is  being 
unwound.  There  will  be  needed  for  each  double  sec- 
tion about  5}/2  oz.  of  No.  34  gauge  single  silk-covered 
magnet  wire.  Make  sure  in  winding  that  the  opera- 
tion is  slow  enough  to  allow  the  wire  to  become  thor- 
oughly saturated,  but  it  must  reach  the  coil  before 
the  wax  has  had  time  to  cool.  You  can  remove  the 
coils  from  the  form  very  easily  if  you  line  the  form 
with  sheets  of  paper  before  you  start  the  winding. 
To  wind  one  of  the  double  sections,  proceed  as  fol- 
lows: Pass  3  or  4  in.  of  the  wire  through  the  opening 
H  in  disk  D1?  Fig.  3,  and  wind  2%  oz.  in  the  regular 
way  upon  the  core  Clf  terminating  the  winding  on 
the  righthand  side  of  the  coil  or  next  to  the  disk  D,. 
When  this  first  coil  is  complete  remove  the  form  from 
the  shaft  and  turn  the  coil  around  and  place  it  back 
on  the  shaft.  The  disk  D2  will  now  be  on  the  left- 
hand  side  of  the  coil  and  the  inside  end  of  the  coil 
winding  will  project  through  the  disk  Dx  on  the  right- 


20  HOW  TO  MAKE  A  WIRELESS  SET 

hand  side.  Carefully  remove  the  disk  D±  and  place 
four  or  five  paper  washers  that  have  been  saturated  in 
paraffin  against  the  side  of  the  coil.  Put  the  core  C2 
in  place  with  its  small  end  to  the  left  and  the  disk  D3  on 
the  outside.  Do  not  replace  disk  Dv  Solder  the  end 
of  the  wire  to  the  inside  end  of  the  first  coil.  Tape 
this  point  thoroughly  and  do  not  use  a  flux  that  will 
destroy  the  wire.  Two  and  three-fourths  ounces  of 
wire  should  now  be  wound  on  the  second  core  and  the 
winding  terminated  at  the  right-hand  side  or  near  the 
disk  D3.  It  no  doubt  would  be  best  to  boil  this  double 
section  in  hot  paraffin  before  it  is  removed  from  the 
form.  If  this  is  done,  do  not  allow  it  to  get  too  cool 
before  the  disks  and  cores  are  removed,  as  the  coil  may 
be  damaged  in  trying  to  separate  them.  These  vari- 
ous sections  may  be  placed  in  linseed  oil  and  allowed 
to  soak,  which  will  add  to  their  insulation  properties. 

When  all  of  the  sections  are  complete  they  should 
be  placed  on  the  outside  of  the  insulating  tube  and 
connected  in  series  in  such  a  way  that  if  a  current  is 
passed  through  them,  it  will  flow  around  the  core  in 
the  same  direction  in  all  of  them.  Seven  or  eight 
paper  washers  saturated  with  paraffin  should  be  placed 
between  each  double  section  as  they  are  placed  on  the 
insulating  tube.  This  completes  the  most  difficult 
part  of  the  work  and  the  next  thing  in  order  will  be 
the  construction  of  a  suitable  case  for  the  coil. 

The  customary  manner  of  constructing  such  a  case 
is  to  make  the  base  in  such  a  form  that  it  will  contain 
the  condenser,  and  the  primary  and  secondary  wind- 
ings can  be  placed  in  a  smaller  case  mounted  upon 
this  base.  The  binding-posts,  which  serve  as  terminals 
for  the  primary,  and  the  vibrator  are  also  mounted 
upon  the  top  of  the  base. 

Cut  from  some  well-seasoned  cherry  or  other  close- 


HOW  TO  MAKE  A  WIRELESS  SET 


21 


grained  wood  two  square  pieces  whose  edge  is  2  in. 
greater  than  the  outside  diameter  of  the  completed 
secondary.  Bore  a  1-in.  hole  through  the  center  of  one 
of  these  pieces  and  a  1-in.  hole  half  way  through  the 
center  of  the  other.  Figure  6  gives  the  dimensions  of 
the  sides  and  the  means  of  fastening  the  ends  and  sides 
together.  The  bottom  can  be  made  of  %-in.  material 
held  in  a  groove  %  in.  wide  and  }4  in-  deep,  cut 
around  the  inside  of  the  frame  just  formed  *4  in-  from 
the  edge.  This  bottom  must  be  put  in  place  before 
the  frame  is  fastened  together.  The  coil  will  also  have 
to  be  put  in  place  before  the  frame  is  fastened  to- 
gether. The  top  can  be  made  from  y^-in.  material 
and  screwed  to  the  top  of  the  frame  with  round- 
headed  brass  screws.  The  corners  can  be  rounded  off 
and  the  box  given  a  nice  finish. 


CHAPTER  III 
THE  CONDENSER 

\  FTER  your  coil  proper  is  completed,  you  can  pro- 
•£*•  ceed  with  the  construction  of  some  of  the  other 
parts  while  you  are  waiting-  for  the  various  -coats  of 
varnish  to  dry  on  the  containing  case.  It  would  be  well 
to  construct  the  condenser  next,  as  it  will  require  some 
time  to  dry  thoroughly,  and  your  coil  will  be  ready  to 
assemble  when  the  remaining  parts  are  complete. 
The  purpose  of  the  condenser  is  to  reduce  the  spark 
at  the  contact  in  the  interrupter,  which  increases  the 
life  of  the  contact  and  at  the  same  time  causes  a  more 
rapid  change  of  current  in  the  primary,  thus  increas- 
ing the  voltage  induced  in  the  secondary.  A  condenser 
consists  of  two  electrical  conductors  separated  by  an 
insulating  material  called  the  dielectric.  The  capacity 
of  a  condenser  depends  upon  the  area  of  the  conduc- 
tors, their  distance  apart,  and  the  kind  of  material  or 
dielectric  separating  the  plates.  You  will  need  at 
least  2.400  sq.  in.  of  exposed  plate  in  a  condenser  suit- 
able for  the  coil  you  are  constructing.  Now,  instead  of 
attempting  to  build  the  condenser  by  using  only  two 
sheets,  you  can  reduce  its  size  by  increasing  the  num- 
ber of  plates  and  arranging  them  as  shown  in  Fig,  1. 
In  determining  the  size  to  use  in  the  condenser  you 
should  bear  in  mind  the  fact  that  it  would  be  desirable 
to  mount  the  condenser  in  the  sub-base  of  your  coil. 
The  upper  containing  case  was  lOi/4  in.  long  and 
approximately  6  in.  wide,  outside  dimensions.  This 
case  should  be  mounted  upon  a  second  case  about  11 


HOW  TO  MAKE  A  WIRELESS  SET  23 

ii  .  wide,  and  17  in.  long.  If  it  is  made  of  %-in.  mate- 
rial, it  will  have  inside  dimensions  of  Q1/^  by  15%  in. 
You  will  want  to  allow  at  least  %-in.  clearance  at 
both  sides  and  one  end,  and  about  3  in.  at  the  other 
end,  where  connections  can  be  made  to  the  condenser. 
This  will  give  you  the  dimensions  of  your  insulating 
sheets  S1/^  in.  wide  and  12  in.  long.  The  tinfoil  that 
you  will  use  as  the  conductors  should  be  cut  into 
sheets  6  by  8  in.  Each  of  these  sheets  will  have  an 
area  of  48  sq.  in.,  and  since  there  is  a  total  of  2,400 
sq.  in.  required  you  will  need  50  sheets.  Twenty-five 
of  these  sheets  will  be  connected  to  one  terminal  and 
the  remaining  25  to  the  other.  You  may  think  that 
there  will  be  50  positive  and  50  negative  plates  re- 
quired, but  each  plate  will  have  both  surfaces  exposed, 
as  shown  in  Fig.  1,  and  only  half  as  much  tinfoil  will 
be  required  in  a  condenser  of  this  type  as  in  one  con- 
sisting of  only  two  large  sheets. 

Procure  a  small  quantity  of  the  very  best  grade  of 
thin  bond  paper  and  cut  about  60  sheets  8%  in.  wide, 
and  13  in.  long.  These  sheets  of  paper  should  be 
boiled  in  paraffin  for  about  one  hour  and  hung  up  to 
drain.  The  paraffin  should  be  real  hot  when  the 
sheets  are  removed  so  that  it  will  drain  off  easily,  re- 
sulting in  a  uniform  thickness  throughout.  When 
they  have  cooled,  cut  ^  in.  from  each  end,  making  the 
sheets  8%  in.  wide  by  12  in.  long.  Now  cut  from  some 
tinfoil  50  pieces  1  in.  wide  and  4  in.  long,  that  are  to 
be  used  in  making  connections  to  the  various  sheets 
or  plates  of  the  condenser. 

It  is  impossible  to  predetermine  the  exact  capacity 
that  will  give  the  best  results,  and  for  this  reason  it 
would  be  best  to  divide  your  condenser  into  a  num- 
ber of  parts,  so  that  the  capacity  can  be  varied.  Such 
an  arrangement  is  shown  in  Fig.  2,  which  amounts  to 


HOW  TO  MAKE  A  WIRELESS  SET  25 

connecting  a  number  of  condensers  in  parallel.  To 
construct  your  condenser  so  that  it  will  correspond  to 
the  above  arrangement,  you  should  divide  the  con- 
nections to  one  set  of  plates  into,  say,  five  groups,  and 
these  various  groups  can  be  connected  to  switches  on 
the  top  of  the  lower  base  of  your  coil.  The  con- 
struction and  connections  of  these  switches  will  be 
taken  up  in  another  chapter. 

To  build  up  your  condenser,  you  should  proceed  as 
follows :  Lay  a  sheet  of  the  paraffin  paper  on  a  smooth 
board  and  place  a  sheet  of  the  tinfoil  in  the  center  of 
this  piece  of  paper.  Take  one  of  the  small  tinfoil  con- 
necting strips  and  put  it  in  place,  as  shown  in  Fig.  3. 
Now  place  a  second  sheet  of  paper  in  place  and  run 
a  squeegee  print  roller  over  it,  applying  quite  a  little 
pressure,  thus  removing  any  air  that  might  exist  be- 
tween the  foil  and  paper.  Place  a  second  sheet  of  tin- 
foil in  place  and  make  the  connection  to  it,  as  shown 
by  full  line  in  Fig.  4.  Continue  in  this  way  until  you 
have  a  total  of  10  sheets  in  place,  the  odd  numbered 
sheets  being  connected  together  at  the  point  T  and 
the  even  numbered  sheets  connected  together  at  the 
point  T!,  Fig.  4.  The  odd  numbers  of  all  the  remain- 
ing sheets  should  be  connected  to  the  common  ter- 
minal T,  and  the  even  numbers  should  be  grouped, 
five  sheets  to  the  group,  and  connected  to  the  ter- 
minals To,  T3,  T4  and  T5.  The  terminals  should  be 
arranged  as  shown  in  Fig.  4,  so  that  there  is  no  likeli- 
hood of  them  coming  in  contact  with  each  other.  Af- 
ter all  the  plates  are  in  place,  lay  three  or  four  sheets 
of  paper  on  the  outside  of  the  condenser  and  clamp  it 
between  two  boards,  as  shown  in  Fig.  5.  The  ter- 
minals formed  of  the  tinfoil  connecting  strips  are  very 
weak  and  can  be  easily  broken  off  when  handling,  so 
that  they  should  be  strengthened  by  folding  some 


26  HOW  TO  MAKE  A  WIRELESS  SET 

pieces  of  very  thin  brass  over  them,  the  end  of  the 
brass  strips  being  slipped  between  the  plates  when  the 
condenser  is  under  construction. 

You  should  now  heat  the  condenser  until  the  par- 
affin softens,  at  the  same  time  increasing  the  tension  in 
the  screws,  and  then  allow  it  to  cool.  It  might  be  well 
to  boil  the  condenser  in  paraffin,  as  that  will  remove 
any  air  that  may  have  collected  between  the  plates. 

You  are  now  ready  to  construct  the  containing  case 
for  the  condenser.  This  case  should  be  about  1  in. 
deeper,  inside  dimension,  than  the  depth  of  the  con- 
denser. The  ends  and  sides  of  this  box  can  be  fas- 
tened together  in  the  same  way  you  made  the  coil 
case,  and  should  be  made  of  %-in.  material.  The 
bottom  should  be  of  such  a  size  that  it  will  project  3A 
to  1  in.  beyond  the  sides  and  ends  of  the  case.  Round 
off  the  corners  as  shown  in  Fig.  6  and  fasten  it  to  the 
frame  with  screws  from  the  under  side.  The  :top.  of 
this  case  should  be  made  of  %-in.  material  with  the 
edges  rounded  and  fastened  to  the  frame  with  brass 
screws.  Fasten  this  piece  to  the  base  of  the  upper 
case  with  screws,  countersinking  their  heads.  The 
position  of  the  upper  case  on  its  base  is  shown  in  Fig. 
7.  After  the  holes  have  all  been  drilled  for  mounting 
the  upper  case,  the  lower  case  should  be  nicely  fin- 
ished to  match  the  upper  one. 

Mount  the  condenser  in  the  end  of  the  lower  case, 
Fig.  7.  Small  blocks  of  wood  can  be  fastened  to  the 
side  of  the  case  and  the  clamps  on  the  condenser  fas- 
tened to  them,  thus  holding  it  rigidly  in  place.  The 
end  of  the  condenser  with  all  the  terminals  on  should 
be  toward  the  open  end  of  the  box.  Mount  upon  the 
end  of  the  top  of  the  lower  case  five  small  switches,  as 
shown  in  Fig.  7.  One  point  on  all  of  these  switches 
should  be  connected  to  one  line,  which  is  to  form  one 


HOW  TO  MAKE  A  WIRELESS  SET  27 

terminal  of  the  condenser.  The  other  points  of  the 
switches  should  be  connected  to  the  terminals  T\,  T2, 
T3,  T4  and  T5  of  the  condenser.  Mount  two  back- 
connected  binding-posts  Bx  and  B2,  Fig.  7,  and  con- 
nect the  terminal  T  of  the  condenser  to  one  of  them 
and  the  common  connection  of  the  switches  to  the 
other.  You  can  now  vary  the  capacity  between  Bx 
and  B2  by  manipulating  the  switches.  By  placing 
these  binding-posts  on  the  base  and  connecting  your 
condenser  to  them  you  can  use  the  condenser  for  other 
purposes  than  across  the  contacts  of  the  vibrator.  Two 
other  binding-posts,  B3  and  B4,  should  be  mounted  be- 
side B!  and  B2  and  they  should  be  connected  to  the 
two  sides  of  the  vibrator  contact.  Connecting  Bx 
and  B3  together,  and  B2  and  B4  together  with  a  piece 
of  wire,  you  will  have  the  condenser  across  the  con- 
tact. Mount  two  more  binding-posts,  BB  and  B6,  a  lit- 
tle larger  than  the  others,  as  shown  in  Fig.  7.  These 
are  to  form  the  terminals  of  the  primary  winding. 

A  single  switching  device  for  the  condenser  is 
shown  in  Fig.  8.  The  various  pieces  of  brass  can  be 
cut  from  some  Vs-in.  sheet  brass  and  fastened  to  the 
board  with  round-headed  screws.  After  they  are  all 
in  place,  a  tapered  reamer  can  be  used  in  cutting  the 
openings  between  them.  The  connections  can  now  be 
made  by  inserting  small  tapered  plugs  in  these  open- 
ings. 

You  can  get  a  better  variation  in  your  capacity  by 
using  the  following  combination  instead  of  the  one 
given  in  the  first  part  of  the  article:  First  section,  14 
sheets;  second  section,  12  sheets;  third  section,  10 
sheets;  fourth  section,  8  sheets  and  the  fifth  section, 
6  sheets.  This  gives  the  same  total  capacity  as  in  the 
previous  case,  from  the  same  total  number  of  plates. 


28  HOW  TO  MAKE  A  WIRELESS  SET 

Cut  from  some  l/$  or  iVin.  brass,  two  pieces  whose 
dimensions  correspond  to  those  of  Fig.  9,  that  are  to 
form  part  of  the  terminals  of  the  secondary  winding. 
These  two  pieces  should  have  ^-in.  holes  drilled  in 
their  centers  and  tapped.  Countersink  them  in  two 
openings  cut  in  the  top  of  the  upper  case  about  8  in. 
apart  and  fasten  them  in  place  with  two  flat-headed 
brass  screws.  A  piece  of  Vs-in.  brass  about  %  in. 
long  should  be  soldered  to  the  under  side  as  shown  in 
Fig.  9.  These  pieces  will  project  through  the  under 
side  of  the  case  top  and  the  terminals  of  the  second- 
ary winding  can  be  soldered  to  them,  leaving  quite  a 
little  slack  in  the  leads.  It  might  be  best  to  make  the 
leads  from  the  coil  to  these  plates  of  wire  insulated  to 
stand  a  high  voltage. 


CHAPTER   IV 
THE  SPARK  GAP  AND  VIBRATOR 

IX   order  to  use   your  spark  coil   for   experimental 
and  wireless  work,  you  will  need  some  form  of  a 
spark  gap.     A  very  simple  one,  that  can  be  mounted 
in  the  plates  attached  to  the  top  of  the  containing  case 
of  the  coil,  can  be  made  as  follows : 

Procure  two  pieces  of  iVin.  brass  rod,  each  about 
5  in.  long,  and  thread  both  ends  to  a  distance  of  about 
J/4  or  j^  in.  Now  turn,  from  some  hard  wood,  two 
pieces  similar  to  the  one  whose  cross-section  is  shown 
in  Fig.  1.  These  pieces  should  have  a  T%-in.  hole 
drilled  through  them  and  the  brass  rods  forced  into 
the  holes,  the  rods  being  held  in  place  by  a  small  brass 
pin,  P,  placed  in  a  hole  drilled  through  the  brass  and 
wood  as  shown  in  Fig.  1.  Obtain  two  brass  balls  about 
1  in.  in  diameter.  Drill  a  TVin-  hole  through  each  of 
them.  At  a  right  angle  to  this  hole  drill  a  second  hole 
half  way  through  i/4  in.  in  diameter,  and  the  remainder 
of  the  way  %  in.  in  diameter.  Tap  the  larger  end  of 
this  hole  so  the  balls  can  be  screwed  on  the  ends  of  the 
brass  rods.  The  smaller  end  should  also  be  threaded 
or  a  small  piece  of  brass  rod  with  threaded  end  sol- 
dered into  it,  that  can  be  used  in  mounting  a  binding- 
post  as  shown  in  Fig.  2.  Obtain  two  pieces  of  iVin. 
brass  rod,  each  about  7  in.  long.  Turn  from  some  hard 
wood  two  handles  as  shown  in  Fig.  3.  Drill  a  iVm- 
hole  in  the  small  end  of  these  handles  to  a  depth  of  at 
least  1  in.  and  force  the  brass  rods  in  place.  The 
other  end  of  the  brass  rods  should  be  threaded  for  a 


HOW  TO  MAKE  A  WIRELESS  SET  31 

short  distance.  These  rods  can  be  passed  through  the 
openings  in  the  balls  and  held  in  place  by  means  of  a 
setscrew,  S,  as  shown  in  Fig.  4.  The  spark  gap  proper 
can  be  adjusted  by  moving  one  or  both  of  these  rods. 
Small  brass  balls  or  pieces  of  zinc  can  be  screwed  on 
the  threaded  ends  of  the  rods  to  form  the  terminals  of 
the  gap. 

The  spark  coil  is  now  complete  with  the  exception 
of  an  interrupter  for  the  primary  circuit.  It  is  impos- 
sible to  use  the  ordinary  alternating  current  in  the 
primary  as  its  frequency  is  too  low  to  produce  the 
required  number  of  reversals  in  the  secondary.  There 
are  three  different  types  of  interrupters  that  are  used 
in  wireless  telegraph  circuits.  These  are  the  elec- 
trolytic, the  mercury  and  the  vibrating  spring  types. 

The  electrolytic  interrupter  consists  of  a  vessel 
containing  a  solution  of  dilute  sulphuric  acid  with  two 
terminals  immersed  in  this  solution.  The  positive 
terminal  or  anode  is  made  of  platinum  and  should 
have  a  surface  of  about  £•  in.  The  negative  terminal 
or  cathode  is  made  of  lead  and  should  have  an  area  of 
something  like  1  sq.  ft.  When  this  interrupter  is  con- 
nected in  series  with  the  primary  of  an  induction  coil 
and  a  source  of  electromotive  force  of  about  40  volts, 
the  circuit  will  be  interrupted,  due  to  the  formation 
and  collapse  of  bubbles  on  the  platinum  electrode. 

The  mercury  interrupter,  which  is  no  doubt  the  one 
most  commonly  used  in  commercial  wireless  stations, 
is  capable  of  making  interruptions  as  high  as  10,000 
per  second.  This  interrupter  consists  of  an  iron  ves- 
sel with  a  vertical  steel  shaft  mounted  in  it,  and  ar- 
ranged to  be  driven  at  a  very  high  speed  by  a  small 
motor.  Attached  to  this  spindle  is  a  small  worm  or 
centrifugal  pump.  Above  this  pump,  there  is  mounted 
on  the  shaft  a  small  nozzle  which  is  connected  to  the 


32  HOW  TO  MAKE  A  WIRELESS  SET 

outlet  from  the  pump.  When  the  spindle  is  rotated, 
the  mercury  is  raised  to  the  nozzle  and*  thrown  out- 
ward against  the  side  of  the  vessel. 

There  is  mounted  in  the  side  of  the  vessel  a  small  in- 
sulated piece  of  metal  which  forms  one  terminal  of 
the  circuit  that  is  to  be  interrupted,  while  the  iron  ves- 
sel forms  the  other  terminal.  By  varying  the  length 
of  the  insulated  segment  and  the  speed  of  the  shaft, 
the  number  of  interruptions  can  be  adjusted  to  the 
desired  value. 

The  vibrating  spring  type  of  interrupter  is  no  doubt 
familiar  to  almost  every  one,  it  being  practically  the 
same  as  the  ordinary  vibrating  bell.  Its  operation  is 
very  satisfactory  when  properly  constructed  and 
adjusted. 

The  construction  of  the  mercury  interrupter  is  rather 
difficult,  and  the  electrolytic  interrupter  requires  40 
volts  or  more  to  operate  it,  so  that  you  had  better  con- 
struct the  vibrating  spring  type. 

The  principle  of  the  spring  interrupter  is  shown  in 
Fig.  5.  When  a  current  is  passed  through  the  circuit 
by  connecting  a  battery  or  some  other  source  of  elec- 
tromotive force  to  the  terminals  Tj  and  T2,  the  iron 
core  I  of  the  coil  C  becomes  magnetized  and  attracts 
the  soft  iron  armature  A,  which  is  mounted  on  the 
spring  S.  When  this  armature  A  is  drawn  toward  the 
iron  core  I,  the  contact  K  is  broken  and  the  core  of  the 
coil  C  becomes  demagnetized,  due  to  the  fact  that  no 
current  is  flowing  in  the  circuit,  thus  allowing  the 
armature  A  to  return  to  its  initial  position  and  again 
complete  the  contact  K,  and  the  same  operation  is 
again  gone  through.  The  winding  C  may  be  the  pri- 
mary winding  of  the  spark  coil  or  it  may  be  an  en- 
tirely different  winding,  as  shown  in  Fig.  6,  the  make- 
and-break  contact  for  the  primary  circuit  being  an 


HOW  TO  MAKE  A  WIRELESS  SET  33 

additional  one  mounted  on  the  spring  S,  as  shown  at 
Kr  This  second  type  of  construction  has  the  advan- 
tage that  it  can  be  used  with  any  coil,  while  the  other 
type  is  mounted  rigidly  to  one  particular  coil. 

The  following  description  is  that  of  a  vibrator  as 
shown  in  Fig.  5,  with  a  little  change,  and  will  serve 
your  purpose.  All  the  care  that  you  may  exercise  in 
the  construction  of  your  coil  will  give  results  far  below 
those  anticipated,  if  the  vibrator  is  of  poor  design. 
The  simple  vibrator  shown  in  Fig.  6  should  have  the 
following  modification  to  make  its  operation  the  best. 
The  make-and-break  contact  is  not  mounted  upon  the 
main  spring  S,  but  upon  a  second  spring,  Sj,  whose 
movement  is  controlled  by  S  through  the  screw  H, 
Fig.  8.-  This  screw  H  passes  through  a  hole  in  S1}  a 
little  larger  in  diameter  than  the  screw  itself,  and 
screws  into  S.  With  this  arrangement,  the  division  of 
the  time  between  the  make  and  break  of  the  circuit  is 
better  made. 

First  procure  some  %-in.  brass  and  cut  a  piece  to 
correspond  to  the  dimensions  given  in  Fig.  7.  Drill 
the  holes  in  this  piece  as  indicated.  Bend  the  part  P 
at  the  dotted  line  until  it  is  at  right  angles  to  the 
remainder  of  the  piece. 

Procure  a  piece  of  spring  brass,  3%  in.  long,  1  in. 
wide  and  ^  in.  thick,  that  is  to  form  the  spring  S,  Fig. 
8.  Round  off  the  upper  end  of  this  piece  and  drill  the 
holes  as  indicated  in  Fig.  9.  The  spring  Sj  should  be 
made  from  some  spring  brass  1/64  in.  thick  and  its 
other  dimensions  should  correspond  to  those  given 
in  Fig.  10.  Drill  the  holes  indicated  in  the  figure 
and  fasten  a  small  piece  of  platinum  in  place  at  the 
point  K. 

Cut  from  a  round  bar  of  soft  iron,  1  in.  in  diameter, 
a  piece  y±  in.  long  and  drill  three  holes  in  it  as  shown 


34  HOW  TO  MAKE  A  WIRELESS  SET 

in  Fig.  11.  These  holes  should  be  threaded  to  take 
machine  screws.  This  piece  can  now  be  mounted  on 
the  upper  end  of  the  spring  S  with  two  small  screws. 

Cut  two  pieces  from  some  %-in.  brass  whose  dimen- 
sions correspond  to  those  given  in  Fig.  12  and  Fig.  13. 
Tap  the  two  holes  in  the  piece  shown  in  Fig.  12.  These 
two  pieces  are  to  be  used  in  mounting  the  springs  S 
and  Si  upon  the  base  of  the  vibrator  as  shown  in 
Fig.  8. 

The  support  for  the  screw  C,  Fig.  8,  can  be  made 
from  some  %-in.  brass  rod.  Take  a  piece  2%  in.  long 
and  thread  one  end  for  a  distance  of  %  in.  Drill  a 
hole  14  in.  from  the  other  end  and  thread  it  to  take 
the  screw  C,  which  should  be  about  -^  in.  in  diameter. 
Drill  a  second  hole  in  the  end  of  the  rod  and  th-read  it 
to  take  the  screw  Q,  Fig.  8,  that  is  to  hold  C  in  place 
after  it  is  once  adjusted.  Another  hole  should  be 
drilled  in  the  threaded  end  of  the  rod  and  a  piece  of 
No.  14  gauge  copper  wire,  about  12  in.  long,  soldered 
into  it  to  be  used  in  making  connections  to  the  vibra- 
tor. Make  two  nuts,  T\  in.  thick,  to  fit  the  thread  cut 
on  the  rod ;  one  rubber  bushing  to  fit  the  large  hole  in 
the  base  of  the  vibrator,  with  an  opening  in  it  l/2  in.  in 
diameter ;  and  two  washers  of  insulation  material,  1 
in.  in  diameter  and  iV  in.  thick,  with  openings  in  them 
}/2  in-  in  diameter.  The  support  for  C  can  then  be  fas- 
tened to  the  base  as  shown  in  Fig.  8.  The  vibrator  is 
now  complete  and  can  be  mounted  on  the  case  of  the 
coil.  A  small  wooden  block  of  sufficient  thickness  to 
place  the  armature  A  on  the  same  level  as  the  end  of 
the  core  of  the  coil  should  be  provided.  Place  the  end 
of  a  piece  of  No.  14  gauge  copper  wire  under  one  of 
the  screws  used  in  mounting  the  base,  and  pass  it 
through  a  hole  in  the  top  of  the  lower  case,  and  solder 
the  other  end  to  the  binding-post  B3.  (See  Fig.  7, 


HOW  TO  MAKE  A  WIRELESS  SET  35 

Chapter  III.)  The  wire  that  was  soldered  to  the  sup- 
port for  C  should  also  be  soldered  to  the  binding-pos* 
B4.  The  primary  winding  is  connected  to  the  binding- 
posts  B5  and  B3.  The  condenser  can  be  connected 
across  the  contact  of  the  vibrator  by  short  pieces  of 
wire  as  previously  described.  Connect  binding-posts 
B4  and  B6  with  a  piece  of  copper  wire. 

Assuming  that  all  connections  have  been  made  as 
directed  and  that  the  construction  of  the  various  parts 
is  complete,  you  can  make  the  final  adjustment  as 
follows:  Connect  12  dry  cells  in  series  to  the  posts  B5 
and  B6.  Adjust  the  spark  gap  to  about  %  in.  and  the 
condenser  to  maximum  capacity.  Turn  the  screw  C 
up  until  it  completes  the  circuit  and  the  vibrator  is 
started.  You  should  now  vary  the  tension  in  the 
screw  H,  the  position  of  the  screw  C  and  the  capacity 
of  the  condenser  until  you  obtain  the  maximum  spark. 
This  adjustment  is  a  matter  of  trial,  becoming  easier 
as  you  operate  your  coil. 


CHAPTER  V 

CONSTRUCTION  OF  TELEGRAPH  KEY  AND 
AERIAL  WIRES 

IT  is  necessary  that  you  have  some  means  of  making 
and  breaking  the  primary  circuit  of  your  spark  coil, 
so  that  there  will  be  impulses  sent  out  from  the  second- 
ary winding  corresponding  to  the  intervals  the  pri- 
mary circuit  is  closed.  An  ordinary  telegraph  key 
will  serve  the  purpose  very  nicely  when  the  primary 
current  is  not  very  large,  and  its  mechanical  operation 
will  be  the  same  as  when  it  is  being  used  on  a  telegraph 
line.  The  following  simple  construction  may,  how- 
ever, be  of  interest  to  those  who  are  constructing  all 
of  the  various  parts  in  preference  to  purchasing  them, 
and  want  a  key  that  will  carry  more  current  and  have 
a  larger  and  better  contact  than  the  ordinary  telegraph 
key. 

Cut  from  a  piece  of  %-in.  sheet  brass,  a  piece  whose 
dimensions  correspond  to  those  given  in  Fig.  1.  Cut 
a  second  piece,  from  some  %-in.  stock,  %  in.  wide  and 
7  in.  long,  and  solder  it  to  the  first  piece,  as  shown  by 
the  dotted  line  D  in  Fig.  1.  File  off  the  edges  and 
ends  of  this  piece,  after  it  is  soldered  in  place,  to  con- 
form to  the  outline  of  the  main  piece.  Solder  two 
other  pieces,  %  in.  wide  and  %  in.  long,  to  the  first 
piece  as  shown  by  the  dotted  lines  A  and  B,  Fig.  1, 
and  insert  two  pieces  of  %-in.  steel  rod,  of  such 
a  length  that  the  ends  project  about  14  in-  These 
projecting  ends  should  be  pointed,  as  shown  in 
Fig.  2.  Drill  a  %-in  hole  at  F.  The  holes  at 
H,  G  and  I  should  be  threaded  for  iVin.  screws 


30  HOW  TO  MAKE  A  WIRELESS  SET 

Cut  another  piece  from  some  Vs-in.  sheet  brass 
as  shown  in  Fig.  3.  The  projecting  arms  P  and 
P!  should  be  bent  up  at  right  angles  to  the  rest  of  the 
piece,  at  the  points  indicated  by  the  dotted  lines.  Be- 
fore bending  these  pieces,  two  holes  should  be  drilled 
in  them,  as  indicated  in  the  figure,  and  threaded  to  take 
TVin.  screws.  Place  a  i3«-in.  brass  thumbscrew,  with 
a  lock  nut  on  it,  in  the  hole  G,  Fig.  1.  The  end  of  this 
screw  should  be  turned  down  to  %  in.  in  diameter  for 
a  distance  of  ^  in.,  and  a  %-in.  washer  of  ic-in-  brass 
made  to  slip  on  the  end.  Secure  two  -fVin.  brass 
bolts,  about  l1/^  in-  long,  that  are  threaded  their  entire 
length  and  each  having  three  or  four  washers  and  four 
nuts.  Place  one  of  these  bolts  through  the  hole  H, 
Fig.  3,  and  fasten  it  in  place  with  a  nut  on  the  under 
side.  The  other  bolt  should  be  fastened  in  the  hole 
H!,  but  insulated  from  the  piece  of  brass.  The  hole 
H!  should  be  drilled  -£•$  in.  in  diameter  and  a  small 
bushing  with  a  -^5 -in.  wall  placed  inside  of  it.  The 
opening  in  the  bushing  should  be  -fa  in.  Place  a  metal 
washer  on  the  bolt  first,  then  an  insulating  washer  and 
slip  it  through  the  hole  and  then  place  on  a  second 
insulating  washer  and  metal  washer  and  lastly  the  nut 
which  will  hold  them  all  in  place.  This  bolt  should, 
however,  have  its  head  filed  off  flat  and  a  small  hole 
drilled  in  the  center,  into  which  a  short  piece  of  plati- 
num wire  is  forced,  and  the  projecting  end  hammered 
down,  thus  giving  a  platinum  surface  considerably 
larger  than  the  area  of  the  wire.  Obtain  two  thumb- 
screws, about  1  in.  long,  that  will  fit  the  threaded  holes 
in  the  arms  P  and  Pj.  Drill  a  aVin.  hole  in  the  end 
of  each  of  these  screws  and  provide  each  with  a  small 
nut  that  will  aid  in  holding  them  in  place  when  their 
final  adjustment  has  been  made. 

The  piece  shown  in  Fig.  3  can  now  be  mounted  on 


HOW  TO  MAKE  A  WIRELESS  SET  39 

a  wooden  base.  This  base  can  be  made  from  a  piece 
of  close-grained  wood,  about  1  in.  thick,  and  its 
dimensions  should  correspond  to  those  given  in  Fig. 
4.  The  holes  in  this  base  should  be  countersunk  on 
each  side  with  a  %-in.  bit  to  a  depth  of  *4  in-  so  that 
the  nuts  on  the  screws  used  in  mounting  will  be  en- 
tirely below  the  surface.  Mount  two  back-connected 
binding-posts  in  the  corners  of  the  base  and  connect 
them  to  the  two  screws  with  wires  placed  in  grooves, 
cut  in  the  under  side  of  the  base. 

A  small  handle  should  now  be  turned  from  a  piece 
of  hard  rubber  or  very  hard  wood  as  shown  in  Fig.  5. 
This  handle  can  be  mounted  on  the  piece  shown  in  Fig. 
1  with  a  %-in.  brass  screw  passed  through  the  hole  F 
from  the  under  side.  Obtain  two  thumbscrews  about 
1  in.  in  length  that  will  fit  the  holes  H  and  I,  Fig.  1 
Each  of  these  screws  should  be  provided  with  a  lock 
nut.  Drill  a  small  hole  in  the  end  of  the  one  you 
intend  to  put  in  the  hole  II  and  rivet  a  piece  of  plati- 
num wire  in  place.  A  screw,  S,  should  be  provided 
on  both  the  pieces  shown  in  Figs.  1  and  3,  to  be  used 
for  electrical  connections. 

Make  a  small  coil  spring  by  winding  a  piece  of  No. 
20  gauge  steel  wire  around  a  i3c-in.  rod.  The  distance 
between  turns  should  approximately  equal  the  diame- 
ter of  the  wire,  and  the  total  length  of  the  spring 
should  be  %  in.  Place  the  end  of  a  piece  of  lamp 
cord  about  4  in.  long  under  the  screw  S,  Fig.  1.  Now 
mount  the  piece  shown  in  Fig.  1  upon  the  piece  shown 
in  Fig.  3,  making  sure  the  coil  spring  is  in  place  before 
the  screws  in  the  supports  P  and  P±  are  given  final 
adjustment.  The  other  end  of  the  piece  of  lamp  cord 
can  now  be  fastened  under  the  screw  S,  Fig.  3.  The 
screws  in  the  holes  H  and  I  can  now  be  adjusted,  giv- 
ing any  desired  movement  of  the  handle  before  the 


40  HOW  TO  MAKE  A  WIRELESS  SET 

contact  is  closed.  The  screw  G  can  be  adjusted  to 
give  any  pressure  of  the  spring  desired. 

It  might  be  well  at  this  point  to  give  the  construction 
of  a  special  switch  to  be  used  in  connecting  the  trans- 
mitting and  receiving  equipment  to  the  aerial  and 
ground.  Figure  6  shows  the  scheme  of  connections, 
the  switch  being  in  the  upper  position  when  you  are 
transmitting  and  in  the  lower  position  when  you  are 
receiving.  The  base  of  this  switch  should  be  made  of 
slate  or  marble  with  its  dimensions  to  correspond  ap- 
proximately to  those  given  in  Fig.  7.  Cut  from  some 
iVin-  sheet  brass,  six  pieces  %  in.  wide  and  4%  in. 
long.  Drill  a  Vs-in.  hole  in  the  center  of  each  of  these 
pieces  and  bend  them  into  the  form  shown  in  Fig.  8. 
Drill  a  %-in.  hole  through  two  of  these  pieces,  after 
they  are  bent,  with  the  dotted  line  shown  in  Fig.  8  ar. 
a  center.  Mount  them  all  on  the  base  with  %-in. 
brass  bolts  about  1%  in.  long.  Each  bolt  should  be 
provided  with  two  additional  washers  and  nuts  to  be 
used  in  making  connections  to  the  switch.  Cut  from 
some  %-in.  sheet  brass,  two  pieces  %  in.  wide  and  7 
in.  long.  Drill  a  %-in.  hole  in  each  end  of  these  pieces, 
-fs-in.  from  the  end.  Round  off  one  end  of  each  of  the 
pieces  to  a  iVm-  radius,  with  the  hole  as  a  center. 
Bend  the  other  end  over  %  in.  from  the  end,  forming 
a  right  angle.  Cut  from  some  hard  rubber,  a  piece  6 
in.  long,  1  in.  wide  and  l/2  in.  thick.  Drill  a  Mrin.  hole 
in  the  center  of  this  piece  and  a  %-in.  hole  %  in.  from 
each  end.  These  various  parts  can  now  be  assembled 
as  shown  in  Fig.  9.  A  small  wooden  handle  can  be 
attached  to  the  rubber  cross  bar  by  means  of  a  ^-in. 
screw,  as  shown  in  Fig.  9. 

Your  sending  equipment  is  now  complete,  with 
the  exception  of  the  aerial.  There  are  numerous 
forms  of  aerials  and  each  is  supposed  to  have  cer- 


HOW  TO  MAKE  A  WIRELESS  SET  41 

tain  advantages  and  disadvantages  peculiar  to 
itself.  It  is  impossible  to  give  the  construction 
of  all  the  different  types,  as  it  would  lead  to 
considerable  confusion  and  you  would  be  at  a  loss 
to  know  just  which  type  to  use.  The  type  described 
below  is  known  as  the  double-ended  "T"  type  and  it  is 
shown  diagrammatically  in  Fig.  10.  First  of  all,  you 
must  select  the  location  for  your  aerial,  bearing  in 
mind  that  it  should  be  well  up  in  the  air  and,  if  possi- 
ble, not  obstructed  by  adjoining  buildings.  You  can 
no  doubt  place  the  aerial  at  the  greatest  height  with 
the  least  trouble  by  supporting  it  on  masts  placed  on 
the  roof  of  the  highest  building  near  your  station.  The 
kind  of  mast  to  use  will  depend  upon  the  requirements. 
In  your  case  a  12  or  15-ft.  mast  will  no  doubt  be 
ample,  and  a  2-in.  iron  pipe  will  do  very  nicely,  as  it 
can  be  easily  handled.  First  obtain  a  good  sized  block 
of  wood  and  cut  it  to  conform  to  the  shape  of  the  roof. 
Then  bore  a  hole  in  it  that  will  take  the  pipe  or  wooden 
mast  you  are  going  to  use  and  fasten  it  in  place  very 
securely.  Two  or  three  guys  should  be  attached  to 
the  upper  end  of  the  mast,  before  it  is  raised.  They 
should  be  attached  to  the  roof  after  the  masts  are 
raised  in  such  a  way  that  they  will  not  interfere  with 
the  raising  or  lowering  of  the  aerial  wires.  Place 
wooden  pins  in  the  upper  ends  of  the  pipes,  and  screw 
on  them  high-tension  insulators.  Fasten  a  small  pul- 
ley to  these  insulators  with  short  pieces  of  seagrass 
line,  and  run  sufficient  ^4-in.  bell  cord  through  the 
pulleys  to  raise  and  lower  the  aerial  wires.  The  dis- 
tance between  the  masts  will  be  governed  by  the  size 
of  roof,  etc. ;  the  greater  this  distance  the  better. 

Cut  from  some  well  seasoned  oak,  two  pieces  l1/^  in. 
thick,  4  in.  wide  and  30  in.  long  and  use  them  in  sus- 
pending the  aerial  wires  between  the  masts.  Cut 


42  HOW  TO  MAKE  A  WIRELESS  SET 

notches  in  these  pieces,  as  shown  in  Fig.  10,  and  tie 
four  porcelain  knobs  to  each  of  the  pieces  with  sea- 
grass  rope.  The  knobs  should  have  a  play  of  at  least 
4  or  5  in.  The  four  equally  spaced  notches  should  be 
used  in  preventing  the  ropes  from  slipping  along  the 
wooden  pieces.  A  good  sized  porcelain  knob  should  be 
tied  in  the  center  of  a  piece  of  seagrass  rope  about  4 
ft.  long,  and  the  ends  tied  around  the  wooden  stretch- 
ers where  the  remaining  two  notches  are  cut.  One 
end  of  the  ropes  that  pass  through  the  pulleys  on  top 
of  the  poles  should  be  tied  to  this  porcelain  knob. 

Run  some  No.  14  gauge  bare  copper  wire  through 
the  insulators  fastened  to  the  stringers,  as  shown  iii 
Fig.  10,  and  fasten  the  wire  in  place.  Solder  a  piece 
of  No.  14  gauge  copper  wire  to  the  points  P  and  P^ 
Fig.  10.  In  the  center  of  this  piece,  solder  another 
piece  that  will  lead  into  your  instrument. 

The  lead-in  wire  should  be  made  as  clear  as  possible, 
and  by  that  is  meant  to  use  just  as  few  insulators  as 
you  can  conveniently  get  along  with,  since  each  tends 
to  dissipate  a  certain  part  of  the  high-tension  current 
and  thus  greatly  lowers  the  efficiency  of  your  sending 
or  receiving.  The  wire  should  be  well  taped  and 
passed  through  a  heavy  porcelain  tube  where  it  goes 
through  the  wall,  or  a  better  way  still  would  be  to 
drill  a  hole  in  the  window  pane  and  pass  the  wire 
through  it. 


CHAPTER  VI 

CONSTRUCTION  OF  A  COHERER  AND 
A  DETECTOR 

T^  ITHER  the  coherer  or  a  detector  may  be  used  in 

••— '  the  receiving  circuit.  When  it  is  desired  to  re- 
ceive with  audible  signals,  the  coherer  is  used  in  com- 
bination with  a  relay  and  battery,  as  shown  in  Fig.  2. 
The  relay  R  controls  a  local  circuit  consisting  of  a  sec- 
ond battery,  B2,  and  a  vibrating  bell,  B.  The  use  of 
the  detector  in  connection  with  a  telephone  receiver,  as 
used  in  receiving,  will  be  taken  up  later. 

One  of  the  easiest  coherers  to  make,  and  one  that 
will  give  excellent  results  when  used  with  a  sensitive 
relay,  is  one  of  the  ordinary  metal  filings  kind.  The 
following  coherer  has  been  found  to  give  excellent  re- 
sults, if  properly  constructed.  A  wooden  base  for  the 
instrument  should  first  be  made  from  some  %-in.  hard 
wood,  about  3%  in.  wide  and  6  in.  long.  Round  off 
the  upper  corners  and  edges  of  this  piece  and  give  it 
two  or  three  coats  of  good  shellac.  Obtain  two  good 
size  binding-posts,  as  shown  in  Fig.  1.  These  binding- 
posts  should  have  holes  in  them  that  will  allow  a  rs\-in. 
brass  rod  to  pass  through.  Mount  the  binding-posts 
on  the  wooden  base  as  shown  in  Fig.  1.  Two  other 
binding-posts,  B3  and  B4,  should  be  mounted  on  the 
ends  of  the  base,  and  back-connected  to  the  first  two. 

These  two  binding-posts  are  to  serve  as  terminals 
for  the  coherer.  Procure  two  pieces  of  g\-in.  brass 
rod,  2  in.  long,  and  amalgamate  one  end  of  each  of 
them  by  first  dipping  the  end  in  acid  and  then  in  mer- 
cury. These  rods  can  now  be  mounted  as  shown.  A 

13 


T      2 


—J-.IH 


HOW  TO  MAKE  A  WIRELESS  SET  45 

glass  tube,  whose  inside  diameter  corresponds  to  the 
outside  diameter  of  the  rods,  should  be  cut  to  such  a 
length  that  it  will  slip  over  the  rods  and  be  almost  in 
contact  with  the  two  binding-posts  Bj  and  B2. 

The  ends  of  the  brass  rods  inside  of  the  glass  tube 
should  be  separated  -LV  to  r$%  in.,  providing  a  small 
chamber,  C,  in  which  the  filings  are  to  be  placed. 
These  filings  can  be  made  from  a  piece  of  nickel,  by 
sawing  it  with  a  medium-toothed  hacksaw.  They 
should  then  be  sifted  through  a  thin  cheesecloth  or  a 
very  fine  sieve  to  separate  the  coarser  filings  from  the 
fine  dust  which  is  not  desired.  A  small  trace  of  silver 
filings  added  to  the  nickel  filings  will  increase  the  sen- 
sitiveness of  the  coherer. 

Assuming  you  have  constructed  the  aerial  at  the 
receiving  end  and  have  made  the  proper  ground  con- 
nection, you  can  put  your  set  in  operation.  You  will, 
of  course,  need  a  300-ohm  relay,  R,  Fig.  2.  This  relay 
will  cost  about  $7,  if  purchased  ready  made,  but  a 
description  of  how  to  build  one  will  be  given  in  one 
of  the  following  chapters  and  its  cost  will  be  consider- 
ably less.  The  completed  connections  at  the  receiving 
end  are  shown  in  Fig.  2.  The  switch  S  is  the  same 
kind  of  a  switch  as  you  installed  at  the  transmitting 
station.  The  brass  rods  of  the  coherer  should  be 
pushed  in  against  the  mass  of  iron  filings  until  the 
armature  of  the  relay  R  is  drawn  up  and  the  bell  B 
starts  to  ring.  Withdraw  the  brass  rods  until  the  bell 
barely  stops  ringing. 

The  distance  between  the  armature  of  the  relay  and 
the  contact  should  not  be  much  more  than  the  thick- 
ness of  a  piece  of  good  quality  wrapping  paper.  The 
adjustment  of  the  spring  controlling  the  armature  of 
the  relay  will  be  found  to  be  rather  tedious. 

It  no  doubt  would  be  best  to  place  the  receiving  and 


46  HOW  TO  MAKE  A  WIRELESS  SET 

sending  equipment  only  10  or  15  ft.  apart  at  first,  using 
short  pieces  of  wire  hung  to  the  ceiling  for  aerials, 
and  the  adjustments  can  be  more  easily  made  than 
when  they  are  quite  a  distance  apart.  The  distance 
can  then  be  increased  and  the  adjustment  continued. 

The  vibrating  bell  must  be  mounted  so  that  the 
hammer  strikes  the  coherer  a  light  tap  when  it  is 
vibrating.  You  could,  if  desired,  make  the  base  of  the 
coherer  large  enough  so  that  the  bell  could  be  mounted 
on  it  permanently.  A  rubber  band  can  be  wrapped 
around  the  hammer  on  the  bell  so  the  striking  will  not 
break  the  glass  rod.  The  operation  of  this  coherer 
has  been  previously  described,  but  it  might  be  well  to 
give  it  again.  The  wave  motion  sent  out  from  the 
aerial  at  the  sending  station  goes  in  all  directions  and 
a  part  of  it  is  caught  by  the  aerial  at  the  receiving  sta- 
tion and  passes  through  the  coherer  to  the  ground. 
The  passage  of  this  current  through  the  filings  in  the 
coherer  causes  them  to  cohere,  this  lowers  the  resist- 
ance of  the  relay  circuit  and  allows  sufficient  current 
to  flow  from  the  battery  to  actuate  the  relay  and  close 
the  bell  circuit. 

The  filings  are  decohered,  of  course,  when  the  ham- 
mer of  the  bell  strikes  the  glass  rod,  and  the  bell  will 
cease  to  operate,  unless  there  is  still  current  through 
the  coherer  which  causes  the  filings  to  immediately 
recohere.  Hence,  the  bell  will  only  stop  ringing  when 
the  current  ceases  to  flow  through  the  coherer.  This 
current  will  flow  so  long  as  the  key  in  the  primary 
winding  of  the  spark  coil  at  the  sending  station  is 
closed,  and  in  this  way  the  dots  and  dashes  of  the  code 
are  sent. 

Two  types  of  detectors  will  be  described — the  "crys- 
tal" and  "electrolytic."  It  has  been  found  that  certain 
metallic  oxides  and  sulphides  possess  the  remarkable 


HOW  TO  MAKE  A  WIRELESS  SET  47 

property  of  conducting  current  a  great  deal  better  in 
one  direction  than  in  the  other,  when  two  dissimilar 
crystals  are  in  contact.  This  condition  is  utilized  in 
the  construction  of  the  so-called  "crystal  detector." 
These  detectors,  when  connected  in  the  aerial  circuit, 
will  transform  the  electrical  oscillations  into  a  pulsat- 
ing current  which  is  unidirectional  and  a  sound  will 
be  produced  in  a  high-resistance  telephone  receiver 
connected  in  parallel  with  the  detector,  without  the 
use  of  a  local  battery. 

The  "pericon"  detector  is  no  doubt  one  of  the  most 
important  crystal  detectors  made,  and  its  construction 
will  be  given  here.  You  should  purchase  a  small 
quantity  of  chalcopyrite,  Cu2SFe2S3,  and  zincite,  ZnO, 
from  a  dealer  in  wireless  telegraph  supplies.  Break  up 
the  largest  pieces  into  small  fragments  and  test  them 
out  by  placing  a  piece  of  each  kind  of  material  in  a 
crystal  detector  as  shown  in  Fig.  3.  The  best  combi- 
nation you  can  obtain  is  selected  and  they  can  be 
mounted  in  a  better  form  of  detector  as  shown  in  Fig. 
5.  The  first  detector  consists  of  nothing  more  than 
two  spring  clips  fastened  to  a  small  block  of  wood  and 
connected  as  shown  in  Fig.  4.  The  most  sensitive 
pair  can  be  determined  by  listening  in  the  telephone 
receiver  T  to  the  signals. 

Cut  from  some  fk-in.  round  brass,  two  pieces  !3/2  in. 
long,  At  and  A2,  Fig.  5.  Drill  a  small  hole  in  one  end 
of  each  of  these  pieces  and  tap  them  to  take  a  Vs-iri- 
screw.  Drill  two  other  holes  through  the  pieces,  % 
in.  from  the  opposite  end,  and  thread  them  to  take  %- 
in.  screws.  Mount  these  pieces  on  a  wooden  base,  2l/2 
in.  wide  and  6  in.  long,  cut  from  some  %-in.  hard 
wood,  with  small  brass  screws  passed  through  the 
base  from  the  under  side.  These  screws  should  be 
countersunk  and  the  brass  pieces  should  be  about  2^ 


48  HOW  TO  MAKE  A  WIRELESS  SET 

in.  apart.  The  holes  in  the  upper  ends  of  these  pieces 
should  be  on  a  line  with  each  other. 

Make  two  rods,  Rt  and  R2,  2l/2  in.  long  and  threaded 
their  entire  length,  to  fit  the  holes  in  the  pieces  Ax  and 
A2.  Two  disks,  Dl  and  D2,  1^4  in.  in  diameter,  should 
be  cut  from  some  y±-'m.  hard  wood  and  fastened  on 
the  ends  of  the  rods  R!  and  R2  to  serve  as  handles  in 
adjusting  the  detector.  Make  from  some  %-in.  brass 
rod  two  cups,  Ci  and  C2.  The  dimensions  are  given  in 
Fig.  6.  Drill  a  small  hole  in  the  bottom  of  these  cups 
and  thread  them  to  fit  on  the  ends  of  the  rods  Rx  and 
R2.  These  holes  should  not  be  drilled  all  the  way 
through  the  bottom  of  the  cups. 

The  two  crystals  you  selected  can  now  be  mounted 
in  these  cups  by  means  of  a  composition  known  as 
Wood's  metal.  This  metal  can  be  purchased  at  a 
supply  store  or  it  can  be  made  by  melting  together 
four  parts  of  bismuth,  two  parts  of  lead,  one  part  of 
tin  and  one  part  of  cadmium.  This  composition  melts 
at  a  very  low  temperature,  something  like  140  deg.  F. 
The  cups  should  be  thoroughly  cleaned  and  then  nearly 
filled  with  this  metal,  and  the  crystals  held  in  place 
until  the  metal  cools  and  hardens.  Two  lock  nuts,  Nt 
and  N2,  should  be  placed  on  the  rods  Rx  and  R,,  as 
shown  in  Fig.  5,  to  hold  the  cups  in  place  after  they 
have  been  once  adjusted.  Two  binding-posts  should 
be  mounted  on  the  base  and  connected  to  the  screws 
on  the  under  side  of  the  base. 

This  detector  can  now  be  connected  in  circuit,  as 
shown  in  Fig.  4,  and  the  final  adjustment  made,  which 
will  remain  for  a  long  period,  unless  it  be  roughly  han- 
dled or  burned  out  by  being  placed  near  a  strong 
transmitter  without  proper  protection.  A  battery,  B, 
should  be  connected  in  series  with  the  telephone  re- 
ceiver to  give  the  b$st  results. 


HOW  TO  MAKE  A  WIRELESS  SET  49 

It  might  improve  the  sensitiveness  of  the  detector 
to  construct  it  so  that  the  crystals  are  held  in  contact 
by  means  of  a  small  spring.  To  do  this,  do  not  thread 
the  hole  in  one  of  the  supports,  but  drill  it  out  so  that 
the  rod  will  move  freely  through  it.  A  small  spring 
can  be  slipped  over  the  rod  between  the  cup  and  the 
support  and  the  lock  nut  used  in  adjusting  the  ten- 
sion in  the  spring,  it  being  placed  on  the  rod  between 
the  end  of  the  spring  and  the  cup. 

An  inexpensive  electrolytic  detector  can  be  made  as 
follows :  The  cup  that  is  to  hold  the  electrolyte  should 
be  made  from  a  piece  of  carbon  rod,  and  its  dimensions 
should  correspond  to  those  given  in  Fig.  7.  Cut  from 
a  rod  of  brass  a  piece  whose  dimensions  correspond  to 
those  given  in  Fig.  8.  Drill  a  small  hole  in  the  bot- 
tom of  this  piece  and  thread  it  to  take  a  %-in.  screw. 
Now  cut  from  some  %-in.  hard  wood  a  piece  5  in. 
long  and  2  in.  wide.  Drill  the  holes  indicated  in  Fig. 
11  and  countersink  them  on  the  under  side.  The  up- 
per edges  of  the  piece  can  be  nicely  rounded  off,  and 
it  should  then  be  given  three  or  four  coats  of  shellac. 
Cut  from  some  %-in.  brass  a  piece  fii/o  in.  long  and  ^ 
in.  wide.  Drill  a  hole  in  the  center  of  this  piece  and 
thread  it  to  take  a  %-in.  screw.  Drill  two  other  ^-in. 
holes,  a/4  in.  from  the  ends. 

Now  bend  the  piece  into  the  form  shown  in  Fig.  9 
and  mount  it  on  the  base  so  that  the  hole  in  its  center 
is  directly  over  the  center  of  the  carbon  cup.  Mount 
two  back-connected  binding-po?ts  with  screws  through 
holes  in  the  ends  of  the  base.  Connect  one  of  these  to 
the  U-shaped  piece  of  brass  and  the  other  one  to  the 
screw  that  holds  the  piece  of  brass,  supporting  the 
carbon  cup,  in  place. 

Obtain  a  low-voltage  lamp  that  has  been  burnt  out 
and  remove  the  pieces  of  platinum  wire  in  the  base  of 


60  HOW  TO  MAKE  A  WIRELESS  SET 

the  lamp.  Be  careful  not  to  break  the  platinum  wires 
loose  from  the  copper  lead-in  wires.  Mount  one  of 
these  pieces  in  the  end  of  a  screw,  S,  Fig.  10,  by  sol- 
dering the  copper  wire  C  in  a  small  hole  drilled  in  the 
end  of  the  screw.  This  screw  should  now  be  placed 
in  the  threaded  hole  in  the  U-shaped  piece  and  pro- 
vided with  a  lock  nut.  The  piece  o-f  platinum  wire  can 
be  raised  and  lowered  with  respect  to  the  carbon  cup. 
The  detector  should  be  connected  in  circuit,  as  shown 
in  Fig.  4,  with  a  battery  in  series  with  the  telephone 
receiver.  Connect  the  positive  pole  of  the  battery  to 
the  binding-post  corresponding  to  the  platinum- 
pointed  screw.  Fill  the  carbon  cup  with  a  solution, 
made  by  adding  one  part  nitric  acid  to  four  parts 
wafer  (always  add  the  acid  to  the  water)  to  within  l/% 
in.  of  the  top.  The  platinum  point  should  just  barely 
touch  the  electrolyte.  Figure  11  shows  the  completed 
detector. 


CHAPTER  VII 
CONSTRUCTION  OF  A  POTENTIOMETER 

THE  successful  operation  of  the  detectors  described 
in  the  previous  chapter  will  depend  upon  some 
device  for  varying  the  current  that  flows  through  the 
detector  from  the  battery.  There  are  ways  by  which 
the  current  can  be  adjusted.  First,  a  suitable  rheostat 
could  be  connected  in  series  with  the  detector  and  the 
current  adjusted  to  the  desired  value  by  changing  the 
total  resistance  of  the  circuit ;  second,  a  potentiometer 
could  be  used.  The  potentiometer  differs  from  the 
rheostat  in  that  it  changes  the  applied  voltage,  which 
in  turn  results  in  a  change  in  the  current,  the  resist- 
ance remaining  constant,  while  the  rheostat  changes 
the  value  of  the  current  by  changing  the  resistance, 
the  voltage  remaining  constant.  The  potentiometer 
is  widely  used  in  connection  with  detectors  employing 
a  local  battery,  it  being  so  sensitive  that  a  very  fine 
adjustment  of  current  can  be  obtained.  The  principle 
of  the  potentiometer  will  be  made  clear  by  reference 
to  Fig,  1.  A  battery,  B,  is  connected  in  series  with  a 
resistance,  AC.  The  direction  of  the  current  flow 
through  this  resistance  is  indicated  by  the  arrow,  and 
any  point  such  as  D  will  be  at  a  higher  electrical  level 
or  potential  than  some  point  such  as  E,  which  is  near 
the  end  C.  If  a  resistance,  R,  be  connected  to  the 
points  D  and  E,  there  will  be  a  current  flow  through 
the  resistance,  due  to  the  potential  difference  between 
D  and  F,  and  the  value  of  the  current  in  amperes  will 
be  equal  to  this  potential  difference,  in  volts,  divided 
by  the  resistance  of  R,  in  ohms.  If  the  position  of  the 
points  D  and  E  on  the  resistance  AC  can  be  changed, 


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HOW  TO  MAKE  A  WIRELESS  SET  53 

the  value  of  the  current  in  R  can  be  varied,  since  the 
potential  difference  between  D  and  E  will  increase  as 
they  are  moved  apart  and  decrease  when  they  are 
moved  toward  each  other.  The  range  of  this  adjust- 
ment would  be  from  the  full  voltage  of  the  battery, 
when  the  points  D  and  E  coincide  with  A  and  C  re- 
spectively, to  zero,  when  they  are  connected  to  the 
same  point  on  the  resistance  AC.  The  resistance  R 
in  Fig.  1  should  be  replaced  by  the  detector  when  the 
potentiometer  is  to  be  used  for  wireless  work. 

A  potentiometer  suitable  for  wireless  work  may  be 
constructed  as  follows :  For  the  base,  obtain  a  piece  of 
oak  or  other  hard  wood,  %  in.  thick,  6  in.  wide  and  18 
in.  long.  Round  off  the  corners  and  edges  on  one  side, 
which  will  add  greatly  to  the  appearance,  and  give  it 
two  or  three  coats  of  good  shellac.  Ten  good  sized 
binding-posts  should  now  be  mounted  on  this  base  as 
shown  in  Fig.  2.  These  posts  should  preferably  be  of 
the  back-connected  type,  and  the  screws  holding  them 
in  place  should  be  countersunk  so  that  they  do  not 
come  below  the  under  side  of  the  base.  Stretch  three 
pieces  of  30  or  32-gauge  bare  german  silver  wire  be- 
tween these  binding-posts  on  the  upper  side  of  the 
board  as  indicated,  in  Fig.  2,  by  the  lines  Lt,  L2  and 
L3.  These  wires  should  not  be  fastened  between  the 
binding-posts  and  the  board,  but  in  the  opening  in  the 
binding-posts,  which  will  raise  the  wires  at  least  J/2  in- 
from  the  board.  Before  fastening  the  binding-posts 
rigidly  to  the  board,  connect  1  and  7,  4  and  5,  2  and  3, 
and  8  and  6  with  a  piece  of  18-gauge  copper  wire 
placed  in  grooves  cut  in  the  under  side  of  the  base. 
These  conductors  are  indicated  by  dotted  lines  in  Fig. 
2.  Obtain  two  pieces  of  lamp  cord,  or  other  flexible 
wire,  about  18  in.  long  and  fasten  one  end  of  these  two 
pieces  under  the  binding-posts  9  and  10.  The  wires 


54  HOW  TO  MAKE  A  WIRELESS  SET 

can  be  passed  down  through  two  holes,  Hx  and  H2, 
drilled  in  the  base  at  the  side  of  the  binding-posts,  and 
the  ends  connected  to  the  binding-posts  on  the  under 
side.  Two  clips  should  be  made  and  fastened  to  the 
other  ends  of  these  wires  for  the  purpose  of  making 
connections  to  the  german  silver  wire. 

These  clips  can  be  made  as  follows :  Cut  from  some 
spring  brass  two  pieces  corresponding  in  dimensions 
and  form  to  that  given  in  Fig.  3.  Bend  these  two 
pieces  into  the  form  shown  in  Fig.  4,  drill  a  small  hole, 
H,  through  the  end  and  solder  the  free  ends  of  the 
wires  in  these  holes. 

In  using  the  potentiometer,  the  battery  should  be 
connected  to  the  posts  7  and  8.  It  would  be  best  to 
place  a  small  switch  in  this  circuit  so  that  the  battery 
can  be  readily  disconnected  when  not  in  use.  The  va- 
riable source  of  potential  corresponds  to  the  termi- 
nals 9  and  10.  The  value  of  the  potential  between 
these  two  binding-posts  can  be  easily  adjusted  by 
moving  one  or  both  of  the  clips  along  the  German 
silver  wire. 

The  various  pieces  of  apparatus  described  thus  far, 
will,  when  properly  connected  and  adjusted,  give  sat- 
isfactory results,  provided  there  are  not  too  many 
wireless  stations  operating  in  the  same  neighborhood. 
The  electrical  waves  sent  out  by  any  wireless  station 
have  a  certain  length  depending,  of  course,  upon  the 
equipment  making  up  the  sending  set.  To  obtain  the 
best  results  in  transmitting  signals  by  wireless,  the 
receiving  and  sending  stations  must  have  such  equip- 
ment and  it  must  be  so  adjusted  that  they  have  the 
same  wave  length.  When  this  condition  is  realized, 
the  stations  are  said  to  be  in  tune.  Nearly  everyone 
has  at  some  time  heard  a  piano  string  vibrate  in  unison 
with  some  note  that  was  sounded  on  another  musical 


HOW  TO  MAKE  A  WIRELESS  SET  55 

instrument.  The  natural  period  of  vibration  of  the 
string  corresponds  to  that  of  the  note  and  it,  as  a 
result,  vibrates  more  than  any  of  the  other  strings, 
which  results  in  its  being  heard.  The  same  condition 
is  true  to  a  greater  or  less  extent  in  wireless  work.  All 
stations  in  the  neighborhood  of  a  station  that  is  send- 
ing will  be  affected,  and  the  effect  increased  as  the 
condition  of  perfect  tuning  is  approached  or  reached. 

There  are  two  important  pieces  of  apparatus,  the 
condenser  and  tuning  coil,  that  are  employed  in  chang- 
ing the  wave  length  of  a  given  station,  and  their  con- 
struction will  be  taken  up  presently.  Before  describ- 
ing the  construction,  however,  it  might  be  well  to  ex- 
plain how  they  are  connected  in  the  receiving  circuit. 
Figure  5  shows  the  connections  of  the  various  parts 
that  compose  a  complete  receiving  set.  The  tuning 
coil  T  consists  of  two  windings  marked  P  and  S ;  VC 
is  a  variable  condenser  (one  whose  electrostatic  capac- 
ity can  be  changed  by  an  adjustment  of  the  plates  com- 
posing the  condenser)  ;  D  is  the  detector ;  TR,  the 
telephone  receiver;  E,  the  potentiometer,  and  B,  the 
battery. 

There  are  two  sliding  contacts  on  the  tuning  coil, 
as  shown  by  Ct  and  C2,  Fig.  5.  The  purpose  of  these 
contacts  is  to  give  a  means  of  changing  the  number  of 
turns  that  are  effective  in  the  primary  or  secondary 
winding,  which  causes  a  change  in  the  value  of  the 
inductance  of  the  two  circuits.  In  addition  to  this 
adjustment,  these  coils  are  usually  so  arranged  that 
their  relative  position  to  each  other  can  be  changed, 
which  varies  the  effect  one  winding  has  on  the  other. 
Such  coils  are  very  often  spoken  of  as  "loose  coupling" 
tuning  coils  or  transformers. 

The  wave  length  of  any  circuit  is  dependent  upon 
its  inductance  and  capacity,  and  the  wave  length  may 


56  HOW  TO  MAKE  A  WIRELESS  SET 

be  varied  by  changing  either  the  inductance  or  the 
capacity  of  the  circuit,  or  both.  The  proper  adjust- 
ment of  the  coil  and  condenser  to  give  the  best  results 
will  be  taken  up  later,  after  you  have  constructed  them 
and  are  ready  to  operate.  Two  types  of  tuning  coils 
will  be  described,  the  construction  being  quite  differ- 
ent while  the  principle  of  operation  is  practically  the 
same  in  both.  The  first  coil  to  be  described  consists  of 
a  primary  winding  on  a  cylinder  of  fiber  with  the  sec- 
ondary winding  on  another  cylinder  of  such  size  and 
so  arranged,  that  it  can  be  moved  in  and  out  of  the 
primary  winding.  One  terminal  of  each  of  these 
windings  should  be  connected  to  a  sliding  or  moving 
contact  so  that  the  number  of  turns  actually  in  use 
can  be  altered  and  the  inductance  thus  changed. 

The  details  of  construction  are  as  follows :  First  ob- 
tain a  piece  of  hard  wood,  %  in.  thick,  5%  in.  wide  and 
16  in.  long.  This  piece  is  to  serve  as  a  base  for  the 
completed  coil.  Round  off  the  upper  edges  to  a  radius 
of  about  y±  in.  and  give  it  two  or  three  coats  of  good 
shellac.  Next  obtain  a  piece  of  good  maple,  %  in. 
thick,  41/2  in.  wide  and  2  ft.  long.  Cut  from  this  board 
two  pieces  of  the  dimensions  given  in  Fig.  6.  Cut  a 
notch,  y±  in.  wide  and  a/4  in.  deep,  in  the  center  of  the 
upper  edge  of  each  of  these  pieces.  Turn  a  groove  iV 
in.  wide  and  %  in.  deep  in  the  surface  of  each  piece 
with  the  point  P,  Fig.  6,  as  a  center.  These  grooves 
should  have  an  outside  diameter  of  3^  in.  In  one  of 
the  pieces,  with  P  as  a  center,  cut  a  circular  opening 
3^  in.  in  diameter.  In  the  other  piece  drill  a  %-in. 
hole  with  P  as  a  center  and  a  second  %-in.  hole  %  m- 
below  P.  The  second  hole  should  not  be  drilled  all 
the  way  through.  Obtain  a  piece  of  insulating  fiber,  iV 
in.  thick,  5i/2  in.  wide  and  10  in.  long.  Bend  this  piece 
into  a  cylinder  5%  in.  long  and  trim  off  the  ends  until 


HOW  TO  MAKE  A  WIRELESS  SET  57 

it  is  of  such  a  diameter  that  it  will  fit  into  the  grooves 
cut  in  the  two  pieces  shown  in  Fig.  6.  You  should 
use  considerable  care  in  trimming  the  edges  so  that 
they  will  fit  against  each  other  perfectly  the  entire 
length  of  the  cylinder.  This  cylinder  should  then  be 
glued  into  the  grooves  in  the  wooden  pieces,  making 
sure  that  these  are  in  line  with  each  other  before  the 
glue  sets,  which  can  be  determined  by  resting  the 
lower  edges  of  the  pieces  on  a  plane  surface.  If  the 
edges  of  the  piece  forming  the  cylinder  do  not  stay  in 
line  with  each  other  they  can  be  held  in  place  in  the 
following  manner:  Wind  around  the  outside  of  the 
cylinder  several  turns  of  wire,  drawing  the  ends  of  the 
piece  of  fiber  into  place.  Then  glue  a  second  piece  of 
fiber  over  the  joint  inside  of  the  cylinder.  Do  not 
remove  the  wire  from  the  cylinder  until  the  glue  in  the 
joint  has  set  thoroughly.  Another  way  of  forming 
the  cylinder,  and  one  that  will  give  good  results,  is  as 
follows :  Obtain  a  sheet  of  thin  fiber,  Sy?  in.  wide,  and 
roll  it  around  a  wooden  cylinder,  3%  in.  in  diameter, 
until  the  wall  of  the  cylinder  thus  formed  is  iV  in. 
thick.  Glue  each  layer  of  the  insulating  fiber  in  place 
as  it  is  wound  on,  and  wind  the  completed  cylinder 
with  several  turns  of  wire  and  allow  it  to  dry. 

When  the  cylinder  has  been  fastened  to  the  wooden 
blocks,  it  is  ready  for  the  winding,  which  is  to  consist 
of  20-gauge  single  cotton-covered  copper  wire.  Drill 
two  iij-in.  holes  in  the  under  side  of  the  cylinder,  % 
in.  from  the  wooden  end  piece  without  the  large  open- 
ing in  it.  These  two  holes  should  be  drilled  about  MJ 
in.  apart.  Pass  the  end  of  the  wire  to  be  used  in 
winding  the  cylinder  down  through  one  of  these  holes 
and  up  through  the  other,  allowing  about  5  in.  of  free 
wire  to  protrude.  You  may  find  it  necessary  to  drive 
a  small  wooden  peg  into  one  of  the  holes  along  the 


53  HOW  TO  MAKE  A  WIRELESS  SET 

side  of  the  wire  to  hold  it  firmly  in  place.  The  end  of 
this  peg  can  be  cut  eft"  even  with  the  surface  of  the  cyl- 
inder and  should  not  project  more  than  %  m-  inside 
of  the  cylinder.  Wind  on  wire  until  you  are  within 
Vi  in.  from  the  other  end  of  the  cylinder.  This  end  of 
the  winding  can  be  terminated  in  the  same  way  the 
other  end  was  fastened,  except  that  no  free  wire  to 
form  a  terminal  is  needed. 

Now,  take  a  small  block,  cover  it  with  a  piece  of  fine 
sandpaper  and  cut  off  the  insulation,  to  a  width  of  ^ 
in.,  on  the  upper  side  of  the  wire  along  the  top  of  the 
cylinder  and  directly  in  line  with  the  two  grooves  in 
the  top  of  the  end  blocks.  The  sliding  contact  is  to 
move  along  a  support  fastened  in  the  two  grooves  and 
make  contact  with  the  various  turns  of  wire  where 
the  insulation  has  just  been  removed. 

The  support  for  this  contact  can  be  made  from  a 
piece  of  brass,  !/4  in.  square,  fastened  in  the  grooves 
with  a  round-headed  brass  screw  in  each  end.  Make 
the  slider  as  follows :  Cut  from  thin  sheet  brass  a  piece 
1  in.  wide  and  liV  in.  long.  Bend  this  piece  around  a 
piece  of  iron,  a/4  in.  square,  in  such  a  way  that  the  joint 
will  come  in  the  center  of  one  side.  Take  a  small 
flat-headed  machine-screw  of  brass,  about  %  in.  long, 
and  solder  it  in  the  center  of  the  side  opposite  the 
joint.  A  small  handle  can  be  turned  from  a  piece  of 
hard  rubber  and  fastened  to  this  screw.  Then  cut 
from  thin  spring  brass  a  piece  *4  in.  wide  and  2*4  in. 
long.  Bend  this  piece  into  the  form  shown  in  Fig.  7 
and  solder  it  to  the  square  brass  tube  on  the  side  where 
the  joint  was  made.  The  complete  sliding  contact 
can  now  be  placed  on  the  brass  support,  and  the  pri- 
mary winding  of  your  tuning  coil  is  complete,  as 
shown  in  Fig.  8,  with  the  exception  of  the  connections 
to  the  binding-posts  that  are  to  serve  as  terminals. 


CHAPTER  VIII 
SECONDARY  WINDINGS 

TO  construct  a  secondary  winding  suitable  to  use 
with  the  primary  winding  described  in  Chapter 
VII  you  would  proceed  as  follows :  Turn  from  a  piece 
of  %-in.  hard  wood  two  disks  3  in.  in  diameter.  Drill 
two  iVin.  holes  in  one  of  these  pieces  as  shown  in  Fig. 
1.  Turn  from  the  same  stock  another  disk  3^  in.  in 
diameter.  The  edges  of  this  disk  should  be  rounded 
off  as  shown  in  Fig.  2.  Now  glue  the  small  disk  with- 
out the  holes  in  it  to  the  larger  disk  and  then  drill  the 
holes  indicated  in  Fig.  3.  The  holes  1  to  8,  inclusive, 
should  be  l/$  in.  in  diameter,  and  their  centers  should 
be  located  on  the  arc  of  a  circle  whose  radius  is  1  in. 
and  its  center  should  be  the  center  of  hole  No.  9.  The 
distance  between  the  centers  of  the  holes  located  on 
the  arc  should  be  %  in.  Hole  No.  9  should  be  fa  in. 
in  diameter. 

Take  eight  brass  machine  screws,  l1^  in.  in  length 
and  y$  in.  in  diameter,  and  file  their  heads  down  as 
shown  in  Fig.  4.  Mount  these  screws  in  the  openings 
1  to  8,  inclusive,  using  a  small  nut  on  the  back  side  to 
hold  them  in  place.  These  screws  are  to  be  connected 
to  various  points  on  the  secondary  winding,  and  a  con- 
tact arm,  which  is  to  form  one  terminal  of  the  wind- 
ing, will  be  arranged  to  move  over  them,  giving  a 
means  of  changing  the  number  of  secondary  turns 
that  are  effective. 

Obtain  a  piece  of  brass  tubing  2  in.  long  with  a  wall 
fa  in.  thick  and  an  opening  fa  in.  in  diameter.  Saw 


HOW  TO  MAKE  A  WIRELESS  SET  61 

three  slots  %  in-  deep  in  one  end  of  this  piece  of  tub- 
ing and  bend  each  of  the  six  parts  thus  formed  in 
toward  the  center  so  that  the  tube  will  fit  very  snug 
over  a  Win-  rod.  Thread  the  other  end  of  the  tube 
to  a  distance  of  1  in.  and  screw  it  into  the  hole  num- 
bered 10  in  Fig.  3.  The  protruding  end  should  be  on 
the  side  corresponding  to  the  smaller  disk. 

Take  a  second  piece  of  the  same  kind  of  tubing,  3  in. 
long,  and  thread  one  end  for  a  distance  of  %  in.  and 
the  other  end  about  !1/2  in-  Saw  three  slots  in  the 
end  with  the  larger  number  of  threads  on  it,  to  a  depth 
of  %  in.  and  bend  the  six  pieces  in  toward  the  center 
of  the  tube,  as  you  did  in  the  previous  case.  Turn  a 
small  handle  from  some  hard  wood  or  rubber  whose 
dimensions  correspond  approximately  to  those  given 
in  Fig.  5.  Drill  a  ^-in.  hole  in  this  handle  as  indi- 
cated by  the  dotted  lines  and  thread  it  so  that  the  han- 
dle can  be  screwed  on  the  end  of  the  brass  tube.  The 
handle  can  then  be  held  in  place  by  means  of  a  small 
setscrew  countersunk  in  a  hole  at  H,  Fig.  5. 

Make  two  nuts  from  some  %-in.  stock  that  will 
screw  on  the  threaded  end  of  the  tube.  Cut  from 
some  thin  spring  brass  a  piece  whose  dimensions  cor- 
respond to  those  given  in  Fig.  6.  Drill  a  %-in.  hole, 
Hj,  in  this  piece.  Make  a  washer  of  some  %-in.  brass 
with  an  opening  VA  in.  in  diameter  and  an  outside 
diameter  of  34  in.  Thread  this  washer  so  that  it  can 
be  screwed  on  the  end  of  the  brass  tube  and  then  sol- 
der it  to  the  contact  arm  A,  shown  in  Fig.  G.  The 
large  opening  in  the  arm  should  correspond  to  the 
opening  in  the  washer.  The  dotted  circle  in  Fig.  G  in- 
dicates the  position  of  the  washer.  After  you  have 
soldered  these  two  pieces  together,  drill  two  £>-'m. 
holes,  Ho  and  H3,  through  both  of  them.  Countersink 
these  holes  in  the  brass  washer  for  flat-headed  screws. 


62  HOW  TO  MAKE  A  WIRELESS  SET 

The  washer  and  arm  can  now  be  screwed  on  the  end 
of  the  tube  and  then  the  wooden  handle  put  in  place 
and  fastened  with  two  small  screws  through  the  holes 
H2  and  H3.  Fastening  the  handle  to  the  washer  and 
arm  after  they  are  all  put  in  place  prevents  the  likeli- 
hood of  an  improper  alinement. 

All  the  parts  are  shown  assembled  in  Fig.  7.  A 
small  spiral  spring,  S,  is  placed  between  the  two  nuts 
Xx  and  N2,  and  the  washer  Wx,  so  that  the  end  of  the 
arm  A  will  always  be  held  against  the  heads  of  the 
screws.  Two  stops  should  be  provided  to  prevent  the 
arm  A  turning  too  far  around  and  dropping  off  the 
heads  of  the  screws.  These  can  be  made  by  driving 
two  small  brass  nails  in  the  wooden  disk  as  shown 
by  N!  and  N2,  Fig.  3. 

The  cylinder  upon  which  the  secondary  winding  is 
to  be  placed  can  be  made  by  winding  some  thin  insu- 
lating fiber  around  a  round  wooden  piece,  3  in.  in  di- 
ameter. This  cylinder  should  be  6  in.  long  and  its  wall 
should  be  iV  in.  thick.  When  the  cylinder  is  com- 
pleted, one  end  can  be  put  over  the  disk  D,  Fig.  7,  and 
fastened  with  three  or  four  small  screws  equally 
spaced  around  the  end  of  the  tube.  The  other  small 
disk  shown  in  Fig.  1  can  be  fastened  in  the  other  end 
of  the  cylinder  in  a  similar  manner,  making  sure  the 
holes  in  the  disks  are  in  line  before  you  fasten  the  last 
one. 

The  secondary  part  of  your  tuning  coil  is  now  com- 
plete, with  the  exception  of  the  winding,  which  should 
be  made  as  follows :  Procure  a  small  quantity  of  No. 
22-gauge  single  cotton-covered  copper  wire.  Drill  a 
small  hole  %  in.  from  the  end  opposite  the  switch. 
Pass  about  10  in.  of  wire  down  through  this  hole  and 
fasten  the  wire  with  a  wooden  peg.  Wind  on  wire 
until  you  have  covered  %  in-  of  the  cylinder,  then  drill 


HOW  TO  MAKE  A  WIRELESS  SET  63 

a  small  hole  as  close  to  the  winding  as  you  can.  Make 
a  loop  in  the  wire  you  are  winding  on  the  cylinder  and 
pass  this  loop  down  through  the  hole  just  drilled  in 
the  cylinder  and  fasten  the  wire  securely.  This  loop 
should  be  at  least  9  in.  long.  Proceed  in  this  wray 
until  you  have  completed  the  winding,  which  should 
consist  of  eight  sections. 

The  end  of  the  winding  farthest  from  the  switch 
should  be  connected  to  the  brass  tube  fastened  in  the 
hole  No.  10,  Fig.  3.  The  loops  in  order,  numbering 
them  from  this  end  of  the  winding,  should  be  fastened 
to  the  screws  placed  in  the  holes  1,  2,  3,  etc.  The  cyl- 
inder must,  of  course,  be  disconnected  from  the  block 
the  screws  are  mounted  on,  in  order  that  the  wires  may 
be  soldered  to  the  screws.  The  secondary  winding 
can  now  be  mounted  inside  of  the  primary  winding  as 
follows : 

Cut  from  your  %-in.  stock  a  piece  whose  dimen- 
sions correspond  to  those  given  in  Fig.  8.  Drill  two 
TVin.  holes,  Hx  and  H2,  in  this  piece.  Mount  this 
piece  on  one  end  of  the  board  that  is  to  form  the  base 
of  the  instrument  with  two  or  three  screws  passed 
through  the  base  from  the  under  side  and  counter- 
sunk. This  piece  should  be  parallel  to  the  end  of  the 
base  and  %  in.  from  the  end.  Mount  the  primary 
winding  on  the  other  end  of  the  board  forming  the 
base,  with  four  screws,  two  in  each  end  piece.  Two 
back-connected  binding-posts  may  be. mounted  in  the 
corners  of  the  base  to  serve  as  terminals  for  the  wind- 
ing. One  of  these  binding-posts  should  be  connected 
to  the  brass  rod  carrying  the  sliding  contact  and  the 
other  to  one  end  of  the  winding  itself.  These  connec- 
tions can  be  passed  down  through  holes  drilled  in  the 
end  of  the  cylinder  and  base  and  placed  in  grooves  on 
the  under  side  of  the  base.  The  end  of  the  frame  for 


64  HOW  TO  MAKE  A  WIRELESS  SET 

the  primary  winding  should  be  1%  in.  from  the  end 
of  the  base. 

Obtain  two  pieces  of  brass  rod  -fV  in.  in  diameter 
and  14%  in.  long.  Thread  both  ends  of  these  pieces 
to  a  distance  of  at  least  ^  in.  Place  a  nut  on  one  end 
of  each  of  these  and  solder  it  in  place  when  the  surface 
of  the  nut  and  the  end  of  the  rod  are  flush.  The  end 
of  the  rod  may  be  filed  down  level  with  the  surface  of 
the  nut  after  they  are  soldered,  which  will  present  a 
better  appearance.  Put  these  two  rods  through  the 
holes  in  the  end  of  the  primary  coil,  then  through 
the  openings  in  the  secondary  coil  and  finally  through 
the  two  openings  through  the  block  on  the  end  of  the 
base.  The  block  must  be  unfastened  from  the  base 
so  that  the  secondary  coil  can  be  put  inside  of  the  pri- 
mary coil.  Two  binding-posts  can  now  be  screwed  on 
the  projecting  ends  of  the  rods  and  they  will  form  the 
terminals  of  the  secondary  winding. 

The  appearance  of  the  coil  can  be  greatly  improved 
by  sandpapering  all  the  various  wooden  parts  and  giv- 
ing them  several  coats  of  good  shellac.  The  two 
windings  can  be  shellacked,  but  care  must  be  used 
not  to  get  any  on  the  primary  where  the  slider  comes 
in  contact  with  the  wire. 

Another  method  of  constructing  a  tuning  coil  that 
will  give  very  good  results  is  as  follows :  First  make  a 
cylinder  41/2  in.  outside  diameter  and  5l/2  in.  long,  with 
a  wall  iV  in.  thick,  by  winding  some  thin  insulating 
fiber  around  a  form  and  gluing  the  various  turns  in 
place.  Now  obtain  two  pieces  of  hard  wood,  6  in. 
long,  6  in.  wide  and  %  in.  thick,  and  cut  them  to  the 
dimensions  given  in  Fig.  9.  Cut  a  groove  in  each  of 
these  pieces  as  shown  by  the  lines  Lt  and  L,.  In  one 
of  these  pieces  drill  %-in.  holes  from  1  to  8  as  indicated 
and  a  i\-in.  hole  at  No.  9.  Obtain  eight  %-in.  machine 


HOW  TO  MAKE  A  WIRELESS  SET  65 

screws,  %  in.  long;  file  their  heads  flat  on  top  and 
mount  them  in  the  holes  1  to  8,  with  a  small  nut  on  the 
back.  Cut  from  some  thin  spring  brass  a  piece  whose 
dimensions  correspond  to  those  given  in  Fig.  10.  This 
piece  should  be  bent  and  a  small  handle  fastened  to  it, 
as  shown  in  Fig.  11.  It  may  now  be  mounted  on  the 
wooden  block  by  means  of  a  iVin.  machine  screw  that 
passes  through  the  hole  No.  10  and  is  fastened  in  the 
back  with  a  lock  nut. 

This  bolt  should  be  fastened  so  that  it  will  not  turn 
when  the  contact  arm  is  moved  over  the  various  con- 
tacts, which  may  be  done  in  the  following  way :  Cut 
from  some  spring  brass  a  piece  %  in.  wide  and  1^  in. 
long.  Drill  two  holes  in  this  piece,  as  shown  in  Fig. 
12.  Now  bend  the  piece  and  solder  it  to  one  of  the 
nuts,  as  shown  in  Fig.  13.  A  small  brass  washer,  W, 
should  be  placed  between  the  contact  arm  and  the  sur- 
face of  the  board  when  the  parts  are  being  assembled. 
Two  stops,  N!  and  N2,  Fig.  9,  should  be  provided  to 
prevent  the  contact  arm  moving  off  the  tops  of  the 
brass  bolts. 

Cut  from  some  %-in.  hard  wood  two  pieces  whose 
dimensions  correspond  to  those  given  in  Fig.  14.  Drill 
a  iVin.  hole  in  the  center  of  each  of  these  pieces. 
These  two  pieces  are  to  be  fastened  to  the  two  end 
pieces  shown  in  Fig.  9  with  screws  that  pass  through 
the  holes  Hj  and  H2.  The  size  of  these  holes  will 
depend  upon  the  size  of  the  screws  you  use. 

Place  the  cylinder  between  the  two  end  pieces  and 
fasten  the  two  pieces  shown  in  Fig.  14  in  place.  Drill 
two  holes  through  the  sides  of  the  cylinder  to  corre- 
spond with  the  hole  H3  in  the  side  pieces.  These 
holes  in  the  cylinder  should  be  at  least  -fs  in-  in  diam- 
eter. The  secondary  coil  is  to  be  placed  inside  of  the 
cylinder  and  mounted  on  a  metal  shaft  that  rests  in  the 


66  HOW  TO  MAKE  A  WIRELESS  SET 

holes  H3  and  provided  with  a  small  handle  on  one  end 
so  the  secondary  can  be  revolved.  Before  taking  up 
the  construction  of  the  secondary,  it  would  be  best  to 
wind  the  primary.  Obtain  a  small  quantity  of  No.  20- 
gauge  single  cotton-covered  copper  wire  and  wind  120 
turns  on  the  cylinder.  This  winding  should  be  divided 
into  two  equal  parts,  there  being  a  space  of  about  % 
in.  between  them,  which  will  permit  the  shaft  that  is  to 
carry  the  secondary  coil  to  pass  through  the  cylinder 
without  coming  into  contact  with  the  winding.  The 
terminals  of  this  winding  can  be  secured  to  the  cylin- 
der as  described  in  Chapter  VII.  The  insulation 
should  be  removed  from  the  wire  and  a  sliding  contact 
constructed  similar  to  the  one  described  for  the  first 
coil.  This  sliding  contact  is  to  form  one  terminal  of 
the  primary  winding,  while  one  end  of  the  winding 
itself  will  form  the  other  terminal.  These  connections 
can  be  made  as  previously  described. 


CHAPTER   IX 
ANOTHER  SECONDARY  WINDING 

A  SECONDARY  winding  that  will  work  with  the 
•**  primary  winding  described  in  Chapter  VIII  can 
be  constructed  as  follows :  Turn  up  a  piece  from  some 
hard  wood,  whose  dimensions  correspond  to  those 
given  in  Fig.  1.  Cut  eight  small  grooves  in  the  outer 
edge  of  this  disk  as  shown.  Drill  a  ^-in.  hole,  H, 
through  the  piece,  making  sure  you  get  it  in  the  exact 
center,  and  then  burn  it  out  with  a  piece  of  square 
iron  so  that  a  piece  y±-in.  square  will  fit  into  it  very 
snug.  Drill  nine  Vs-in.  holes,  numbered  1  to  9,  in  the 
cylinder,  1/2  in.  from  the  outer  edge  and  parallel  to  its 
axis.  Now  drill  nine  more  small  holes  through  the 
partitions  between  the  various  grooves  and  connecting 
with  the  nine  drilled  parallel  with  the  axis  of  the  cyl- 
inder. The  position  of  these  holes  is  shown  in  the 
diagram  by  dotted  lines. 

Obtain  a  small  quantity  of  No.  22-gauge  single  cot- 
ton-covered copper  wire  for  use  in  winding  the  sec- 
ondary coil.  Pass  one  end  of  the  wire  down  through 
hole  Ht  and  out  through  hole  No.  1,  allowing  at  least 
8  in.  of  free  wire,  to  be  used  in  making  connections. 
Wind  22  turns  of  wire  in  groove  Gj  and  at  the  point 
where  the  hole  H2  was  drilled,  and  cut  a  groove  in  the 
partition  so  that  the  wire  can  pass  over  irito  the  next 
groove,  G2.  Remove  the  insulation  from  the  wire  di- 
rectly over  the  hole  H2  and  solder  a  piece  of  wire  6  or 
7  in.  long  to  the  main  wire.  Pass  the  short  piece  of 
wire  down  hole  H2  and  out  through  hole  No.  2.  Con- 
tinue in  this  way  until  you  have  completed  the  eight 
sections  of  the  secondary.  All  of  the  terminals  of  the 

67 


I  jf  =  -^  =  I 

p±3£±ij  I 


HOW  TO  MAKE  A  WIRELESS  SET  69 

various  sections  should  be  brought  out  at  the  same  end 
of  the  cylinder.  Give  the  completed  coil  several  coats 
of  good  shellac. 

The  shaft  upon  which  the  secondary  coil  is  to  be 
mounted  and  the  handle  used  in  turning  it  can  be  made 
as  follows :  Obtain  a  piece  of  brass,  a/4  in.  square  and 
6%  in.  long.  Turn  the  ends  of  this  piece  down  to  a 
diameter  of  %  in.  as  shown  in  Fig.  2.  Turn  from 
some  iV-in-  brass  two  washers  %  in.  in  diameter  and 
drill  in  each  of  them  a  V^-'m.  hole  and  slip  one  of  them 
over  each  end  of  the  shaft.  The  wooden  side  pieces 
will  rest  against  these  washers,  which  will  prevent  the 
shaft  moving  endwise.  In  turning  down  the  end  of 
the  shaft,  make  a  good  square  shoulder  for  the  wash- 
ers to  rest  against.  Thread  the  right-hand  end,  as 
shown  in  the  figure,  to  a  distance  of  %  in.  Remove  the 
two  side  pieces  and  the  end  with  the  switch  on  it  from 
the  primary  and  slip  the  shaft  in  place,  having  placed 
the  secondary  coil  inside  of  the  primary  with  the 
square  hole  through  it  in  line  with  the  two  holes  in  the 
side  of  the  cylinder.  The  core  of  the  secondary  can  be 
fastened  to  the  shaft  with  a  screw  placed  in  the  hole 
S,  Fig.  1,  which  will  prevent  it  from  moving  along  the 
shaft.  The  core  of  the  secondary  should  be  turned 
so  that  the  ends  of  the  terminals  project  from  it  toward 
the  open  end  of  the  cylinder.  Now  solder  the  ter- 
minal from  hole  No.  2  to  the  back  end  of  contact  No. 
1,  Fig.  3  (Chapter  VIII),  and  the  other  terminals  in 
regular  order  to  the  contacts  2  to  8  inclusive.  The 
wire  projecting  from  hole  No.  1  is  to  form  one  ter- 
minal of  the  secondary  and  the  center  of  the  switch 
is  to  form  the  other  terminal.  A  piece  of  wire,  7  or  8 
in.  long,  should  have  one  end  fastened  to  the  bolt 
holding  the  switch  in  place.  Two  small  holes  should 
be  drilled  in  the  lower  part  of  the  cylinder  and  as  near 


70  HOW  TO  MAKE  A  WIRELESS  SET 

the  end  block  as  possible  and  the  two  wires  forming 
the  terminals  passed  down  through  them.  Be  sure  to 
have  sufficient  slack  in  the  wire  from  the  secondary 
so  that  it  will  not  interfere  with  the  free  motion  of 
the  core  about  its  axis.  The  coil  can  now  be  assem- 
bled and  it  is  ready  to  mount  on  a  base  with  the  ex- 
ception of  a  small  handle  to  fasten  on  the  end  of  the 
shaft.  The  dimensions  of  a  handle  suitable  for  this 
purpose  are  given  in  Fig.  3.  A  small  screw  placed  in 
the  hole  S  will  serve  to  hold  the  handle  from  turning 
on  the  shaft  after  it  is  once  in  place. 

The  base  can  be  made  from  a  piece  of  hard  wood 
%  in.  thick  and  should  be  7^-j  in.  wide  and  8%  in. 
long.  The  upper  edges  and  corners  of  this  piece 
should  be  rounded  off  and  it  then  given  two  or  three 
coats  of  shellac.  Four  back-connected  binding-posts 
should  be  mounted  on  the  base,  one  in  each  corner, 
and  the  leads  from  the  primary  and  secondary  wind- 
ings connected  to  them.  The  leads  should  be  placed 
in  grooves  cut  in  the  under  side  of  the  base. 

A  small  condenser,  constructed  in  such  a  way  that 
its  capacity  may  be  varied  from  zero  to  its  full  value 
(known  as  a  variable  condenser),  will  be  needed  in 
order  that  the  best  results  may  be  obtained  when  you 
are  using  the  tuning  coils  previously  described.  Any 
condenser  consists  of  two  or  more  conducting  plates 
separated  from  each  other  by  some  kind  of  insulating 
material  that  is  called  the  dielectric.  The  capacity  of 
such  a  combination  of  plates  and  dielectric  is  meas- 
ured in  terms  of  the  quantity  of  electricity  the  con- 
denser will  store  when  it  is  subjected  to  a  certain  elec- 
trical pressure.  A  condenser  is  said  to  have  unit  ca- 
pacity (one  farad)  when  a  unit  quantity  of  electricity 
(one  coulomb)  will  raise  its  potential  one  volt.  The 
capacity  will  depend  upon  the  area  of  the  plates  ex- 


HOW  TO  MAKE  A  WIRELESS  SET  71 

posed  to  each  other,  their  distance  apart  and  the  kind 
of  material  separating  them.  In  the  following  de- 
scription of  a  variable  condenser,  use  is  made  of  the 
fact  that  the  capacity  varies  directly  as  the  area  of  the 
plates  exposed,  the  distance  between  them  and  the 
dielectric  remaining  constant.  The  construction,  in 
brief,  consists  of  a  stationary  set  of  plates  mounted  in 
a  pile,  all  separated  from  each  other  the  same  distance 
and  electrically  connected.  This  combination  of 
plates  is  to  form  one  terminal  of  the  condenser.  A 
second  set  of  plates,  of  practically  the  same  area,  are 
mounted  in  a  pile  and  electrically  connected,  the  dis- 
tance bet\yeen  them  being  the  same  as  the  distance 
between  those  forming  the  stationary  pile.  This  sec- 
ond set  of  plates  is  mounted  in  such  a  way,  with  respect 
to  the  first  set,  that  the  plates  forming  the  second  or 
variable  set  may  be  moved  into  the  intervening  spaces 
between  the  stationary  plates.  The  extent  of  this  move- 
ment will  determine  the  area  of  the  plates  forming  one 
combination  that  is  influenced  by  the  plates  of  the 
other  combination,  which  together  with  their  distance 
apart  and  the  kind  of  dielectric  used  determines  the 
capacity  of  the  condenser.  The  dielectric  used  in  such 
a  condenser  is  usually  air  and  their  distance  apart  is 
determined  by  the  mechanical  construction.  The  con- 
denser may  be  constructed  as  follows : 

You  will  need  18  pieces  of  metal  cut  to  the  dimen- 
sions given  in  Fig.  4,  which  are  to  form  the  stationary 
plates  of  the  condenser.  These  plates  may  be  cut 
from  some  thin  sheet  aluminum,  about  .04  in.  thick. 
Considerable  care  should  be  exercised  not  to  bend  the 
sheets  out  of  shape  when  you  are  cutting  them.  Drill 
three  %-in.  holes,  Hlt  H?  and  H3,  in  each  of  the  18 
plates  as  shown.  In  addition  to  the  above  18  plates, 
two  more  will  be  needed  to  complete  the  number  re- 


72  HOW  TO  MAKE  A  WIRELESS  SET 

quired  for  the  stationary  part  of  the  condenser.  Cut 
from  some  iVin.  sheet  aluminum  two  whose  dimen- 
sions correspond  to  those  given  in  Fig.  5.  Drill  in 
each  of  these  plates  the  holes  indicated.  Bend  the 
projecting  part  P  down  at  right  angles  to  the  main 
part  of  the  plate  at  the  dotted  line  D^  After  this  bend 
has  been  made,  again  bend  P  at  the  dotted  line  D2  so 
that  it  is  parallel  to  the  main  plate.  A  cross-section 
through  the  plate,  after  these  bends  have  been  made,  is 
shown  in  Fig.  6.  These  plates  are  to  be  mounted  in  a 
pile  as  follows :  Obtain  a  piece  of  hard  rubber  8  by 
8  in.  and  about  ^  in.  thick.  Drill  in  this  piece  of  rub- 
ber the  holes  indicated  in  Fig.  7.  Obtain  three  pieces 
of  %-in.  brass  rod,  7  in.  long,  and  thread  them  to  a  dis- 
tance of  about  ll/2  in.  at  one  end  and  about  1  in.  at  the 
other.  Provide  each  of  these  rods  with  six  %-in.  nuts. 
Procure  about  300  small  washers  approximately  .05 
in.  in  thickness.  The  outside  diameter  of  these  wash- 
ers should  be  about  %  in.  and  there  should  be  a  %-in. 
hole  through  them.  Place  a  nut  on  each  of  the  rods 
on  the  end  with  the  larger  number  of  threads,  then 
put  on  ten  of  the  small  washers,  and  put  the  ends  of 
the  rods  down  through  the  holes  H,,  H2  and  H3,  shown 
in  Fig.  7.  Put  two  nuts  on  the  ends  of  the  rods  that 
protrude  through  the  base  and  screw  these  nuts  up 
until  the  last  one  is  just  flush  with  the  end  of  the  rod. 
The  first  nuts  you  placed  on  the  rods  may  now  be 
tightened,  thus  fastening  the  rods  rigidly  to  the  base. 
Now  place  one  of  the  plates,  cut  to  the  dimensions 
given  in  Fig.  5,  on  the  three  rods  with  the  projecting 
part  P  toward  the  base.  Place  three  washers  on  each 
rod  and  then  one  of  the  eighteen  similar  plates.  Con- 
tinue in  this  way  until  all  of  the  18  plates  are  in  place. 
After  all  the  18  plates  are  in  place  put  the  remaining 
heavy  plate  on  the  rods  with  the  projecting  portion 


HOW  TO  MAKE  A  WIRELESS  SET  ?3 

above  the  main  part  of  the  plate.  Put  a  washer  on 
each  of  the  rods  and  then  a  nut  and  draw  all  of  the 
plates  down  in  place.  You  may  experience  some  dif- 
ficulty in  not  having  all  the  plates  the  same  distance 
apart,  owing  to  a  variation  in  the  thickness  of  the 
washers.  This  can  be  prevented  by  making  your  own 
washers  and  cutting  them  all  from  the  same  sheet  of 
metal. 

Cut  from  a  piece  of  sheet  rubber  or  fiber  about  % 
in.  in  thickness,  a  circular  disk  7  in.  in  diameter  and 
drill  three  %-in.  holes  in  it,  as  shown  in  Fig.  8,  so  it 
will  slip  down  on  the  three  rods  that  support  the  sta- 
tionary plates.  A  fourth  hole,  H4,  %  in.  in  diameter 
should  be  drilled  in  the  center  of  this  plate,  and  three 
other  small  ones  as  shown.  This  plate  cannot  be  put 
in  place  until  the  movable  portion  of  the  condenser 
has  been  completed,  which  may  be  constructed  as  fol- 
lows: 

Cut  19  plates  from  the  same  sheet  metal  you  used 
for  the  18  plates,  to  the  dimensions  given  in  Fig.  9. 
Drill  a  T$-in.  hole,  H,  in  each  of  these  plates.  Obtain 
a  piece  of  i^-in.  brass  rod  about  5l/2  in.  long.  Thread 
one  end  of  this  rod  to  a  distance  of  1  in.  and  the  other 
end  to  a  distance  of  about  2  in.  Provide  four  y$-in. 
nuts  that  will  fit  the  threads  on  this  rod.  Cut  from 
some  hard  rubber  two  pieces  whose  dimensions  corre- 
spond to  those  given  in  Fig.  10.  Drill  a  hole  through 
these  pieces  and  thread  them  so  that  they  may  be 
screwed  on  to  the  ends  of  the  i^-in.  brass  rod.  Place 
two  of  the  nuts  on  the  end  of  the  rod  with  the  fewer 
number  of  threads  on  it  and  screw  on  one  of  the  pieces 
of  rubber  with  the  larger  end  toward  the  nuts.  Allow 
%  in.  of  the  rod  to  protrude  through  the  rubber  and 
screw  one  of  the  nuts  down  on  top  of  the  rubber,  which 
will  serve  to  hold  it  in  place.  The  end  of  the  rod 


74  HOW  TO  MAKE  A  WIRELESS  SET 

should  be  rounded  off  before  the  rubber  piece  is  put  in 
place.  Slip  one  of  the  pieces  on  the  rod  shown  in  Fig. 
9,  and  then  three  washers  and  then  another  piece,  etc., 
until  all  the  pieces  are  in  place.  The  washers  sepa- 
rating the  pieces  should  be  the  same  thickness  as 
those  separating  the  stationary  plates  and  these  should 
be  about  %  in.  in  diameter,  which  will  give  a  larger 
bearing  surface.  Place  a  nut  on  the  rod  after  the 
plates  are  all  in  place  and  screw  it  down.  The  posi- 
tion of  the  plates  along  the  rod  should  be  adjusted  so 
that  they  will  move  into  the  spaces  between  the  sta- 
tionary plates  when  the  piece  of  rubber  on  the  lower 
end  of  the  rod  is  placed  in  the  hole  in  the  projection 
on  the  heavy  lower  plate  of  the  stationary  set.  This 
adjustment  can  be  made  by  moving  the  two  nuts  hold- 
ing the  plates  in  place  up  or  down  on  the  rod.  All 
of  these  plates  should  be  placed  directly  over  each 
other  when  they  are  finally  fastened  in  place 

The  upper  piece  of  rubber  should  be  placed  on  the 
rod  so  that  it  will  fit  in  the  hole  in  the  projection  in 
the  upper  heavy  plate.  A  nut  should  be  placed  on  the 
rod  before  the  piece  of  rubber  and  it  can  be  screwed 
up  against  the  piece  of  rubber  after  adjustment. 

The  upper  heavy  plate  must  be  removed  so  that  the 
movable  portion  of  the  condenser  may  be  put  in  place. 
The  large  rubber  disk  can  now  be  fastened  on  the 
three  rods  by  means  of  two  nuts  on  each  rod,  one 
above  and  one  below  the  disk.  Before  fastening  the 
disk  in  place  cut  from  a  sheet  of  brass,  or  bend  ft  from 
a  strip,  a  piece  similar  to  that  shown  in  Fig.  12,  and 
fasten  it  to  the  rubber  disk  as  shown  by  the  dotted 
lines  in  Fig.  8,  by  means  of  three  screws  that  pass  up 
through  the  disk  from  the  under  side.  File  in  the 
upper  side  of  this  piece  a  number  of  grooves  as  shown 
by  radial  lines  in  Fig.  12. 


HOW  TO  MAKE  A  WIRELESS  SET 


75 


Cut  from  a  piece  of  thin  spring  brass  a  piece  similar 
to  that  shown  in  Fig.  11  and  fasten  it  to  the  surface 
F  of  a  handle  similar  to  that  shown  in  Fig.  3.  Screw 
this  handle  down  on  the  upper  end  of  the  rod  that  sup- 
ports the  movable  plates  until  the  arm  A.  Fig.  11, 
rests  upon  the  piece  P,  Fig.  8.  The  arm  A  should  rest 
on  the  piece  P  very  near  its  end  when  the  movable 
plates  are  outside  of  the  stationary  plates.  If  the  end 
of  the  rod  protrudes  through  the  handle,  you  can  file 
it  off  until  it  is  just  flush  with  the  surface  of  the  han- 
dle. Any  adjustment  of  the  movable  plates  will  be 
maintained  by  the  arm  engaging  in  the  grooves  on  the 
surface  of  the  piece  P,  Fig.  8. 

Mount  on  the  base  a  small  piece  of  spring  brass  so 
that  it  will  press  up  against  the  lower  end  of  the  rod, 
supporting  the  movable  plates,  with  a  slight  pressure. 
Mount  two  binding-posts  in  the  corners  of  the  base 
and  connect  one  of  them  to  the  spring  and  the  other 
to  one  of  the  rods  supporting  the  stationary  plates. 
These  binding-posts  will  forni  the  terminals  of 'your 
condenser. 


CHAPTER    X 
CONNECTIONS  FOR  SETS 

THE  various  tuning  devices  described  in  the  previ- 
ous chapters  were  designed  to  be  used  as  a  part 
of  the  equipment  at  the  receiving  station.  The  purpose 
of  these  devices  was  to  enable  the  operator  at  the  re- 
ceiving station  to  adjust  his  equipment  so  that  it  would 
respond  to  a  certain  frequency  wave.  The  frequency 
of  these  waves  is  determined  by  the  construction  and 
adjustment  of  the  transmitting  station.  If  the  induc- 
tion coil  be  connected  to  the  aerial  as  shown  in  Fig.  1, 
the  frequency  of  the  wave  such  a  combination  would 
transmit  would  be  fixed  and  there  would  be  no  way  of 
changing  the  frequency  unless  some  change  were  made 
in  the  construction  of  the  aerial.  This  condition  of 
affairs  would  confine  the  frequency  at  which  such  a 
station  could  transmit  to  a  single  value.  It  is  often 
desirable,  however,  to  have  some  means  by  which  the 
frequency  of  the  waves  sent  out  by  the  transmitting 
station  can  be  varied.  With  this  adjustment,  the  sta- 
tion can  communicate  with  several  different  stations 
whose  receiving  equipment  is  adjusted  to  respond  to 
waves  of  different  frequencies.  The  theory,  in  brief, 
upon  which  the  construction  and  adjustment  of  the 
transmitting  station  is  based,  is  as  follows : 

If  a  condenser  be  discharged  through  a  circuit  con- 
sisting of  resistance  alone,  the  current  that  exists  in 
the  circuit  at  any  instant  after  the  circuit  is  closed  will 
be  equal  to  the  ordinate  (the  distance  from  the  hori- 
zontal line  to  the  curve)  of  the  curve  at  that  particular 

77 


78  HOW  TO  MAKE  A  WIRELESS  SET 

instant.  It  will  be  seen  from  the  curve  shown  in  Fig. 
2  that  the  current  is  a  maximum  at  the  instant  the  cir- 
cuit is  closed  and  decreases  in  value  as  the  time  of  dis- 
charge increases.  The  decrease  in  the  value  of  this 
current  is  very  rapid,  it  requiring  only  about  .0001  of 
a  second  for  the  current  in  a  circuit,  consisting  of  a 
2-micro-farad  condenser  charged  to  a  potential  of  100 
volts  and  connected  to  a  resistance  of  10  ohms,  to  de- 
crease to  about  .06  ampere.  The  current  in  this  cir- 
cuit, just  at  the  instant  the  circuit  is  closed,  would  be 
10  amperes,  it  being  equal  to  the  voltage  of  the  con- 
denser divided  by  the  resistance  of  the  circuit,  or  100 
divided  by  10  equals  10  amperes.  If  the  condenser  be 
connected  to  a  coil  of  wire  and  allowed  to  discharge 
through  it,  the  current  will  not  follow  a  curve  such  as 
that  shown  in  Fig.  2,  where  it  is  represented  as  dying 
away  gradually,  but  it  will  oscillate  back  and  forth  in 
the  circuit,  as  shown  in  Fig.  3.  The  circuit  in  the  sec- 
ond case  possesses  a  peculiar  property  called  induct- 
ance, which  is  in  reality  a  contracted  form  for  the  word 
"self  induction"  and  corresponds  to  what  might  be 
called  "electrical  inertia."  It  is  a  property  of  the  cir- 
cuit which  tends  to  prevent  any  change  in  the  value 
of  the  current  in  the  circuit.  Thus,  if  the  current  in  a 
circuit  is  increasing  in  value,  there  will  be  an  increase 
in  the  magnetic  field  associated  with  the  circuit,  and 
this  increase  in  the  field  strength  will  produce  an  elec- 
tromotive force  in  such  a  direction  as  to  oppose  the 
increase  in  the  value  of  the  current.  If  the  current 
decreases  in  strength,  the  reverse  action  will  take 
place,  the  induced  electromotive  force  tending  to  pro- 
long the  current  or  rather  prevent  a  change  in  its 
value. 

Oscillations  will  not  always  take  place  in  a  circuit 
in  which  there  is  inductance  and  capacity,  but  certain 


HOW  TO  MAKE  A  WIRELESS  SET  79 

conditions  as  to  the  relation  between  the  value  of  the 
inductance  and  the  capacity  must  be  fulfilled  before 
there  will  be  an  oscillating  current  produced.  The 
"fundamental  equation"  of  wireless  telegraphy  gives 
this  relation  as  follows :  There  will  be  oscillations  in  a 
circuit  provided  the  resistance,  in  ohms,  is  not  greater 
than  the  square  root  of  four  times  the  inductance,  in 
henries  (The  henry  is  the  unit  of  inductance.  Refer 
to  some  book  on  electricity  for  its  definition.),  divided 
by  the  capacity  of  the  condenser,  in  micro-farads. 

An  instrument  termed  a  transmitting  helix  is  em- 
ployed at  the  transmitting  station  to  furnish  the  in- 
ductance necessary  to  produce  the  oscillations  in  the 
circuit,  and  a  variable  condenser  will  supply  the  ca- 
pacity. In  addition  to  furnishing  the  inductance,  the 
helix  acts  as  a  step-up  auto  transformer  and  increases 
the  voltage  of  the  high-frequency  currents  in  the  aerial 
circuit.  The  helix  consists  of  a  piece  of  large  copper 
conductor  bent  into  the  form  of  a  spiral  of  quite  a 
number  of  turns  and  supported  on  a  suitable  frame. 

The  operation  of  the  helix  can  be  shown  by  refer- 
ence to  Fig.  4,  which  shows  the  induction  coil,  con- 
denser, helix  and  aerial  all  connected.  The  current 
from  the  secondary  winding  of  the  induction  coil 
passes  through  the  condenser  C  and  the  part  of  the 
helix  that  is  connected  between  the  points  A  and  B. 
This  results  in  the  condenser  becoming  charged,  and 
it  would  discharge  through  the  secondary  winding  of 
the  induction  coil  if  it  were  not  for  the  inductance  of 
the  induction  coil  secondary,  which  has  a  large  counter 
electromotive  force  produced  in  it,  and  in  order  to 
discharge,  the  circuit  is  completed  through  the  spark 
gap.  When  the  adjustment  of  the  capacity  and  in- 
ductance in  the  circuit  composed  of  the  condenser  C, 
the  part  of  the  helix  between  the  points  A  and  B  and 


80  HOW  TO  MAKE  A  WIRELESS  SET 

the  spark  gap  is  properly  made,  there  will  be  electrical 
oscillations  set  up  in  the  circuit.  The  high-frequency 
oscillations  in  the  part  of  the  helix  from  A  to  B  pro- 
duce a  strong  magnetic  field  about  all  the  turns  in  the 
helix  and,  as  a  result,  the  electromotive  force  between 
the  points  D  and  B  will  usually  be  greater  than  that 
between  the  points  A  and  B.  The  part  of  the  helix 
between  D  and  B  can  be  thought  of  as  a  secondary 
winding,  which  in  this  case  consists  of  the  turns  on 
the  helix  between  the  points  D  and  B.  The  electro- 
motive force  in  this  secondary  part  causes  electrical 
oscillations  to  be  sent  out  into  the  aerial  and  ground. 

The  circuit  composed  of  the  condenser,  spark  gap 
and  the  turns  in  the  helix  between  the  points  A  and  B, 
is  called  the  "closed  circuit,"  while  the  circuit  com- 
posed of  the  aerial,  ground  and  the  tur'ns  in  the  helix 
between  D  and  B  constitutes  what  is  called  the  "open 
circuit." 

A  helix  suitable  to  be  used  with  the  other  equip- 
ment previously  described  can  be  made  as  follows : 
Cut  from  a  piece  of  maple,  or  other  hard  wood,  about 
%  in.  thick,  two  circular  pieces  10  in.  in  diameter. 
Cut  eight  grooves  in  each  of  these  pieces,  as  shown 
in  Fig.  5,  to  a  depth  of  %  in.  Cut  from  some  7/s~'in- 
hard  wood,  eight  pieces  %  in.  wide  and  8  in.  long. 
These  pieces,  called  "struts,"  are  to  form  the  supports 
for  the  wire  and  should  have  a  number  of  grooves  cut 
in  them  in  which  the  wire  can  be  placed.  In  order  to 
mark  the  proper  location  of  these  grooves,  you  should 
proceed  as  follows :  First  obtain  a  board  about  3  ft. 
long  and  1  ft.  wide.  One  edge  of  this  board  should  be 
reasonably  straight.  Draw  eight  lines  perpendicular 
to  the  straight  edge  of  the  board  spacing  them  3^f  in. 
apart.  Now  tack  the  eight  struts  to  the  board  with 
some  very  small  brads,  placing  the  edge  of  one  of  them 


HOW  TO  MAKE  A  WIRELESS  SET  81 

along  each  of  the  lines  and  the  ends  even  with  the 
edge  of  the  board.  The  pieces  must  all  be  on  the  same 
side  of  the  lines.  Figure  6  shows  the  eight  pieces  fas- 
tened to  the  board  B.  Number  the  pieces  1  to  8,  start- 
ing with  the  left-hand  one  as  No.  1.  Measure  off  a  dis- 
tance of  1  in.  from  the  lower  end  of  piece  No.  1  and  a 
distance  of  Hf  in.  from  the  lower  end  of  piece  No.  8, 
marking  the  points  on  the  pieces  with  a  light  lead-pen- 
cil mark.  Make  10  other  marks,  %  in.  apart,  on  each 
of  these  pieces  above  those  just  made.  Now,  with  the 
use  of  a  straightedge,  draw  a  line  across  all  of  the 
pieces  as  shown  in  Fig.  6,  by  means  of  the  broken 
lines.  Draw  another  set  of  lines  %  i°-  above  each  of 
the  first  set  and  parallel  to  them.  Cut  11  grooves  in 
each  of  the  struts  to  a  depth  of  %  in.,  using  the  lines 
that  are  y$  in.  apart  on  the  pieces  as  marks  for  the 
sides  of  the  grooves. 

When  all  the  grooves  have  been  cut,  the  pieces  can 
be  fastened  to  the  two  circular  pieces,  or  heads,  shown 
in  Fig.  5.  Drill  a  %-in.  hole  in  each  end  of  the  eight 
struts,  !/2  in.  from  the  end.  Obtain  16  round-headed 
brass  screws  about  %  in.  in  diameter  at  the  shank  and 
2  in.  long.  The  eight  struts  should  now  be  fastened 
in  the  grooves  in  the  two  heads  by  means  of  the  brass 
screws.  The  struts  should  be  placed  in  regular  order 
around  the  circular  pieces,  starting  with  No.  1  and 
ending  with  No.  8.  When  the  cage  thus  formed  has 
been  completed,  give  it  several  coats  of  shellac. 

The  frame  of  the  helix  may  be  raised  above  the  sur- 
face upon  which  it  is  to  rest  by  means  of  three  wooden 
legs  similar  to  those  shown  in  Fig.  7.  These  legs  can 
be  fastened  at  three  equally  spaced  places  on  the  under 
side  of  the  cage  by  means  of  two  screws  that  pass 
through  the  holes  Hx  and  H2  into  the  tower  head  of 
the  helix. 


HOW  TO  MAKE  A  WIRELESS  SET  83 

About  32  ft.  of  No.  8-gauge  bare  copper  wire  will  be 
needed  for  the  winding  on  the  helix.  Drill  a  Vg-in. 
hole  through  the  lower  end  of  strut  No.  1  in  the  center 
of  the  groove.  Pass  about  3  in.  of  the  wire  through 
the  hole  and  proceed  with  the  winding,  placing  the 
wire  in  the  grooves  in  the  struts,  using  considerable 
care  in  not  bending  the  wire  too  abruptly  where  it 
passes  over  the  various  struts.  A  %-in.  hole  can  be 
drilled  in  the  center  of  the  upper  groove  in  strut  No. 
8  and  the  end  of  the  wire  passed  through  it,  which  will 
serve  to  hold  the  winding  in  place. 

The  two  ends  of  the  winding  should  be  given  one 
half  turn  around  the  struts  through  which  they  pass 
and  two  small  binding-posts  may  be  soldered  to  them 
to  serve  as  terminals  to  the  coil. 

The  connections  A  and  D,  Fig.  4,  can  be  made  by 
means  of  two  small  clips  soldered  to  the  ends  of  flexi- 
ble cords.  These  clips  can  be  made  as  follows :  Turn 
out  of  some  hard  rubber  two  pieces  whose  dimensions 
correspond  to  those  given  in  Fig.  8.  Cut  from  some 
spring  brass  two  pieces  similar  to  the  ones  shown  in 
Fig.  9.  Drill  a  small  hole,  H,  in  this  piece,  as  shown 
in  Fig.  9,  and  bend  it  into  the  form  shown  in  Fig.  10. 
Obtain  two  pieces  of  good  rubber-covered  wire,  about 
3  ft.  long  (preferably  a  flexible  conductor),  pass  one 
end  through  the  hole  H  in  the  rubber  handle,  as  shown 
in  Fig.  8,  and  solder  it  in  the  hole  H  in  the  piece  of 
spring  brass  shown  in  Fig.  10.  Pull  the  wire  back 
through  the  handle  and  force  the  spring  clip  in  place. 
After  the  clip  has  been  forced  into  place,  a  piece  of 
rubber  should  be  cut  and  placed  between  the  two  parts 
P!  and  P2  as  shown  in  Fig.  10. 

In  the  operation  of  a  transmitting  station  such  as 
that  shown  in  Fig.  4,  it  is  desirable  to  know  the  value 
of  the  current  in  the  conductor  to  the  aerial  so  that  the 


84  HOW  TO  MAKE  A  WIRELESS  SET 

proper  adjustment  of  the  circuit  may  be  made  in  order 
that  the  energy  transmitted  by  the  station  be  a  maxi- 
mum. The  only  instrument  suitable  for  the  measure- 
ment of  this  current  is  a  hot-wire  ammeter.  Its  ope- 
ration depends  entirely  upon  the  heating  effect  of  a 
current  and  its  indications  are  not  influenced  on  ac- 
count of  the  direction  of  the  current  changing.  If  a 
hot-wire  ammeter  of  suitable  range  is  obtainable,  the 
adjustment  of  the  sending  station  can  be  made  as  fol- 
lows :  Connect  the  ammeter  in  circuit  as  shown  in  Fig. 
11.  Place  the  two  movable  clips  Q  and  C2  on  the 
helix  near  its  center,  close  the  key  in  the  primary  cir- 
cuit and  note  the  ammeter  reading.  Move  both  the 
clips  along  the  helix  until  you  find  the  point  giving  a 
maximum  indication  on  the  ammeter.  Fasten  the 
clip  C2  at  this  point  and  then  move  Q  until  you  get 
the  maximum  reading  on  the  ammeter.  When  this 
adjustment  is  secured,  the  two  circuits  are  in  "tune." 
Figures  1,  4  and  12  show  three  different  types  of 
sending  sets,  while  figures  12,  13  and  14  show  three 
different  types  of  receiving  sets.  Figure  1  shows  an 
"open-circuit  sending  set,"  Figure  4  a  "tuned  sending 
set"  and  Figure  12  a  "loop  aerial  system"  for  sending. 
Figures  13  and  14  show  two  "tuned  receiving  sets" 
and  Figure  12  a  "loop  aerial  system"  for  receiving. 
The  letters  in  the  above  diagrams  refer  to  the  follow- 
ing apparatus : 


B— Battery 

C— Fixed  Condenser. 

D— Detector. 

G— Ground. 

H— Helix. 

K— Key. 

L— Lead  to  Aerial. 

P— Potentiometer. 

R— Telephone  Receiver- 


A  G— Anchor  Gap. 
AS— Aerial  Switch. 
1C— Induction  Coil. 
LT— Loose  Coupled  Coil. 
PC— Plate  Condenser. 
SG— Spark  Gap. 
ST— Straight  Tuning  Coil. 
VC— Variable  Condenser- 


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Metal  Spinning 

By  PROF.  F,  D.  CRAWSHAW 

Assistant  Dean,  College  of  Engineering 
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f^rm rente:-     Chapter  i,  The  Laihe  and  Its  Uses;   ^,  Tools;   3,  The 
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handbook  gives  brief  and  practical  direc- 
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